Remove unused imports from depot_tools scripts

This change removes some unused imports across depot_tools scripts.

Change-Id: Ic1112065ad1020fc4a3573d3fb9f137915ef439c
Bug: 1475776
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4812185
Auto-Submit: Aravind Vasudevan <aravindvasudev@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

git_cache.py

@@ -8,7 +8,6 @@
 from __future__ import print_function
 
 import contextlib
-import errno
 import logging
 import optparse
 import os