bot_update: don't remove git cache's .lock files.

bot_update and gclient invokes git cache with --ignore-locks.
This should also reduce noisiness of bot_update stdout.

R=iannucci@chromium.org

Bug:
Change-Id: I6baf9e5665d244d08aab58a7d3327e6c3204b2b5
Reviewed-on: https://chromium-review.googlesource.com/791536Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -575,14 +575,6 @@ def _maybe_break_locks(checkout_path, tries=3):
 def git_checkouts(solutions, revisions, shallow, refs, git_cache_dir,
                   cleanup_dir):
   build_dir = os.getcwd()
-  # Before we do anything, break all git_cache locks.
-  if path.isdir(git_cache_dir):
-    git('cache', 'unlock', '-vv', '--force', '--all',
-        '--cache-dir', git_cache_dir)
-    for item in os.listdir(git_cache_dir):
-      filename = os.path.join(git_cache_dir, item)
-      if item.endswith('.lock'):
-        raise Exception('%s exists after cache unlock' % filename)
   first_solution = True
   for sln in solutions:
     sln_dir = path.join(build_dir, sln['name'])