Revert of git_cache.py: Clobber git repos if the config is corrupt (patchset #4 id:60001 of https://codereview.chromium.org/2031773002/ )

Reason for revert:
Suspected to break builds: http://crbug.com/618124

Original issue's description:
> git_cache.py: Clobber git repos if the config is corrupt
>
> See https://build.chromium.org/p/tryserver.chromium.win/builders/win10_chromium_x64_rel_ng_exp/builds/1801/steps/bot_update/logs/stdio
> [Mirror] https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.win%2Fwin10_chromium_x64_rel_ng_exp%2F1801%2F%2B%2Frecipes%2Fsteps%2Fbot_update%2F0%2Fstdout
>
> Sometimes repos become corrupt and bot_update fails spectacularly.  This usually happens due to the process being interrupted during a fetch. Git
> cache should be able to recover from this by clobbering.
>
> BUG=
>
> Committed: https://chromium.googlesource.com/chromium/tools/depot_tools/+/b48b126ffd2d43e8fa7897c8174e2688e113af4d

TBR=estaab@chromium.org,phajdan.jr@chromium.org,hinoka@google.com,hinoka@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review-Url: https://codereview.chromium.org/2049463003