Revert "commit-msg: Insert Change-Id at start of trailers"

This reverts commit 653f8b71

.

Reason for revert: This requires git-2.15 which is much newer than
repo itself requires.  Lets pull it until we can figure out something
on the Gerrit side.

Bug: https://crbug.com/gerrit/12546
Change-Id: I5148f8a9cab5f0c305c020e31627b4af88cd5c95
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/263012
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>