There are many functions in git_cl that appear unused, although
it's possible that some of them are used in other places, where
git_cl may be imported as a library.

This CL just removes some of the private ones, which at least
were not supposed to be used from other places.

Things removed here:

 - _get_commiter_timestamp
 - _git_amend_head
 - GitCookiesChecker._get_usernames_of_domain
 - constants MERGE_BRANCH, CHERRY_PICK_BRANCH;
   the comment says that these are supposed to
   be used in CMDland, but apparently they aren't.
   contextlib which is only used by this, is also
   removed
 - CodereviewSettingsFileMock in git_cl_test;
   it is not used anywhere in the test, and I
   assume that nobody's importing and using
   git_cl_test...

git_cl_test still passes, git cl seems to still work

Change-Id: Ib8eee0dfb181c8cda04c0894d2daea52b3ad42ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1818621Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>