Revert "gclient: Add a GCLIENT_PY3 env var to allow users to execute gclient using Python 3."

This reverts commit e656e98831.

Reason for revert: Defaults to Python 3 unless GCLIENT_PY3 is 1...

Original change's description:
> gclient: Add a GCLIENT_PY3 env var to allow users to execute gclient using Python 3.
> 
> Bug: 1008202
> Change-Id: I23dd70b72089a09c2a7cdc74e2a63c135a924ebe
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1825996
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,apolito@google.com

Change-Id: I8c63d8c9e0be84b1286275142fab1885597b1551
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1008202
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1828330
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

gclient

@@ -19,12 +19,4 @@ if [[ "#grep#fetch#cleanup#diff#setdep#" != *"#$1#"* ]]; then
   esac
 fi
 
-# Ensure that "depot_tools" is somewhere in PATH so this tool can be used
-# standalone, but allow other PATH manipulations to take priority.
-PATH=$PATH:$base_dir
-
-if [[ $GCLIENT_PY3 != 1 ]]; then
-  PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/gclient.py" "$@"
-else
-  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/gclient.py" "$@"
-fi
+PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/gclient.py" "$@"

gclient.bat

@@ -12,9 +12,4 @@ call "%~dp0update_depot_tools.bat" %*
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-IF "%GCLIENT_PY3%" == "1" (
-  python "%~dp0gclient.py" %*
-) ELSE (
-  :: TODO(1003139): Use vpython3 once vpython3 works on Windows.
-  python3 "%~dp0gclient.py" %*
-)
+python "%~dp0gclient.py" %*