Revert "[depot_tools] Support both gn paths in gn.py"

This reverts commit df8c52a549.

Reason for revert: Found bugs

Original change's description:
> [depot_tools] Support both gn paths in gn.py
>
> Build in support for both buildtools/<platform>/gn and
> buildtools/<platform>/gn/gn preemptively.
> This will allow the libfuzzer builders in https://chromium-review.googlesource.com/c/chromium/src/+/5474162 to succeed.
>
> Bug: b/328065301
> Change-Id: I97b401cb1b3339cfa7962f60b891be05baac75d5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5479888
> Reviewed-by: Joanna Wang <jojwang@chromium.org>
> Commit-Queue: Stephanie Kim <kimstephanie@google.com>

Bug: b/328065301
Change-Id: I2480e587ce314f7856edc7011fed945e01924371
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5482563
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

gn.py

@@ -33,33 +33,22 @@ def PruneVirtualEnv():
     ])
 
 
-# TODO(b/328065301): Once chromium/src CL has landed to migrate
-# buildtools/<platform>/gn to buildtools/<platform>/gn/gn, only return
-# gn/gn path.
-def findGnOptionsInPath():
+def findGnInPath():
     env_path = os.getenv('PATH')
     if not env_path:
         return
     exe = 'gn'
     if sys.platform in ('win32', 'cygwin'):
         exe += '.exe'
-    path_options = []
     for bin_dir in env_path.split(os.pathsep):
         if bin_dir.rstrip(os.sep).endswith('depot_tools'):
             # skip depot_tools to avoid calling gn.py infinitely.
             continue
-        for path in [
-                os.path.join(bin_dir, 'gn', exe),
-                os.path.join(bin_dir, exe),
-        ]:
-            if os.path.isfile(path):
-                path_options.append(path)
-        return path_options
+        gn_path = os.path.join(bin_dir, exe)
+        if os.path.isfile(gn_path):
+            return gn_path
 
 
-# TODO(b/328065301): Once chromium/src CL has landed to migrate
-# buildtools/<platform>/gn to buildtools/<platform>/gn/gn, only return
-# gn/gn path.
 def main(args):
     # Prune all evidence of VPython/VirtualEnv out of the environment. This
     # means that we 'unwrap' vpython VirtualEnv path/env manipulation.
@@ -74,35 +63,29 @@ def main(args):
     # downloaded by cipd in the projects DEPS.
     primary_solution_path = gclient_paths.GetPrimarySolutionPath()
     if primary_solution_path:
-        old_gn_path = os.path.join(primary_solution_path, 'third_party',
-                                   'gn' + gclient_paths.GetExeSuffix())
-        new_gn_path = os.path.join(primary_solution_path, 'third_party', 'gn',
-                                   'gn' + gclient_paths.GetExeSuffix())
-        for path in [new_gn_path, old_gn_path]:
-            if os.path.exists(path):
-                return subprocess.call([path] + args[1:])
+        gn_path = os.path.join(primary_solution_path, 'third_party', 'gn',
+                               'gn' + gclient_paths.GetExeSuffix())
+        if os.path.exists(gn_path):
+            return subprocess.call([gn_path] + args[1:])
 
     # Otherwise try the old .sha1 and download_from_google_storage locations
     # inside of buildtools.
     bin_path = gclient_paths.GetBuildtoolsPlatformBinaryPath()
     if not bin_path:
-        gn_path_options = findGnOptionsInPath()
-        for path in gn_path_options:
-            if os.path.exists(path):
-                return subprocess.call([path] + args[1:])
+        gn_path = findGnInPath()
+        if gn_path:
+            return subprocess.call([gn_path] + args[1:])
         print(
             'gn.py: Could not find checkout in any parent of the current '
             'path.\nThis must be run inside a checkout.',
             file=sys.stderr)
         return 1
-    old_gn_path = os.path.join(bin_path, gclient_paths.GetExeSuffix())
-    new_gn_path = os.path.join(bin_path, 'gn', gclient_paths.GetExeSuffix())
-    paths = [new_gn_path, old_gn_path]
-    for path in paths:
-        if os.path.exists(path):
-            return subprocess.call([path] + args[1:])
-    print('gn.py: Could not find gn executable at: %s' % paths, file=sys.stderr)
-    return 2
+    gn_path = os.path.join(bin_path, 'gn' + gclient_paths.GetExeSuffix())
+    if not os.path.exists(gn_path):
+        print('gn.py: Could not find gn executable at: %s' % gn_path,
+              file=sys.stderr)
+        return 2
+    return subprocess.call([gn_path] + args[1:])
 
 
 if __name__ == '__main__':