Revert "Support main branch in bot_update"

This reverts commit e7a31f42.

Reason for revert: has typo and doesn't work if typo is fixed

Original change's description:
> Support main branch in bot_update
>
> Change-Id: I58e88e9505779c309ac0af9d5b78b45feaa75543
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2528669
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ib6b266250b854e2554249b956766214be84ec522
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2643961Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

recipes/recipe_modules/bot_update/api.py

@@ -450,7 +450,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
       return 'HEAD'
 
     target_ref = self.m.tryserver.gerrit_change_target_ref
-    if target_ref in ['refs/heads/master', 'refs/heads/master']:
+    if target_ref == 'refs/heads/master':
       return 'HEAD'
 
     return target_ref