bot_update & gclient on bots instruct git cache locks
to be ignored, so there is no need to clean them up.

R=hinoka@chromium.org

Change-Id: I81c52cfcd44a42d13b641847439ac92053a7bc98
Reviewed-on: https://chromium-review.googlesource.com/791652Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>