Remove explicit refs/heads/master revision from gerrit solution

Recipe-Nontrivial-Roll: infra
Bug: 1469734
Change-Id: I7c84383525351798c270cdc3b2f239493f1349e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4743948
Reviewed-by: Arthur Bello <artbello@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

recipes/recipe_modules/gclient/config.py

@@ -486,7 +486,6 @@ def gerrit(c):
   s = c.solutions.add()
   s.name = 'gerrit'
   s.url = 'https://gerrit.googlesource.com/gerrit.git'
-  c.revisions['gerrit'] = 'refs/heads/master'
 
 @config_ctx(includes=['gerrit'])
 def gerrit_plugins_binary_size(c):