Remove non-CIPD Git installation code path. This changes the stable Git
version to use the Git package and removes legacy Git support.

Note that this is different from legacy (non-CIPD-managed) bootstrap.
This will be removed in a future patch.

This is effectively a roll of bleeding-edge into stable.

BUG=chromium:740171
TEST=local

Change-Id: Ia320efd7866f6c11e8aa18d6ce6157614b44fcab
Reviewed-on: https://chromium-review.googlesource.com/567764
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>