Remove unused functions, and use scm.GIT where possible to reduce
duplication.

Change-Id: I24f05e7b3ae04743e97b6665ee668f44d6acc294
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2116938Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>