Revert "Return HEAD when ref is refs/heads/main"

This reverts commit cbdf0c1e41.

Reason for revert: 
Broke tint bots

From log:
https://logs.chromium.org/logs/tint/buildbucket/cr-buildbucket.appspot.com/8864003131025797600/+/steps/bot_update/0/stdout 

Bot page:
https://ci.chromium.org/p/tint/builders/try/linux-clang-rel-x64/b8864003131025797600?

Original change's description:
> Return HEAD when ref is refs/heads/main
>
> R=​ehmaldonado@chromium.org
>
> Change-Id: I67b40cf48f0f9880e0f055a83d768bbefd704892
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2528118
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>

TBR=dpranke@google.com,ehmaldonado@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Bug: 1147494
Change-Id: Ieb21e15161ccc267f5d453a0196b1f0a2b43f366
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2529700
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

recipes/README.recipes.md

@@ -58,7 +58,7 @@ Recipe module to ensure a checkout is consistent on a bot.
 
 Wrapper for easy calling of bot_update.
 
-— **def [deapply\_patch](/recipes/recipe_modules/bot_update/api.py#525)(self, bot_update_step):**
+— **def [deapply\_patch](/recipes/recipe_modules/bot_update/api.py#524)(self, bot_update_step):**
 
 Deapplies a patch, taking care of DEPS and solution revisions properly.
     
@@ -91,7 +91,7 @@ Args:
   * enforce_fetch: Enforce a new fetch to refresh the git cache, even if the
     solution revision passed in already exists in the current git cache.
 
-— **def [get\_project\_revision\_properties](/recipes/recipe_modules/bot_update/api.py#502)(self, project_name, gclient_config=None):**
+— **def [get\_project\_revision\_properties](/recipes/recipe_modules/bot_update/api.py#501)(self, project_name, gclient_config=None):**
 
 Returns all property names used for storing the checked-out revision of
 a given project.
@@ -109,7 +109,7 @@ Returns (list of str): All properties that'll hold the checked-out revision
 
 &emsp; **@property**<br>&mdash; **def [last\_returned\_properties](/recipes/recipe_modules/bot_update/api.py#44)(self):**
 
-&mdash; **def [resolve\_fixed\_revision](/recipes/recipe_modules/bot_update/api.py#453)(self, bot_update_json, name):**
+&mdash; **def [resolve\_fixed\_revision](/recipes/recipe_modules/bot_update/api.py#452)(self, bot_update_json, name):**
 
 Sets a fixed revision for a single dependency using project revision
 properties.

recipes/recipe_modules/bot_update/api.py

@@ -424,7 +424,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
 
     If there's no Gerrit CL associated with the run, returns 'HEAD'.
     Otherwise this queries Gerrit for the correct destination ref, which
-    might differ from refs/heads/master or refs/heads/main.
+    might differ from refs/heads/master.
 
     Args:
       * cfg: The used gclient config.
@@ -434,8 +434,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
 
     Returns:
         A destination ref as understood by bot_update.py if available
-        and if different from refs/heads/master or refs/heads/main, returns
-        'HEAD' otherwise.
+        and if different from refs/heads/master, returns 'HEAD' otherwise.
     """
     # Ignore project paths other than the one belonging to the current CL.
     patch_path = self.m.gclient.get_gerrit_patch_root(gclient_config=cfg)
@@ -445,7 +444,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
       return 'HEAD'
 
     target_ref = self.m.tryserver.gerrit_change_target_ref
-    if target_ref in ['refs/heads/main', 'refs/heads/master']:
+    if target_ref in ['refs/heads/master', 'refs/heads/master']:
       return 'HEAD'
 
     return target_ref