sync: finish marking REPO_AUTO_GC=1 as deprecated.

The wrong revision of the change was submitted as
d7935538.

Change-Id: I6f3e4993cf40c30ccf0d69020177db8fe5f76b8c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/353934


Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Sam Saccone <samccone@google.com>