upload: drop support for drafts

Draft CLs were replaced by private/wip CLs in Gerrit instead years ago.

Change-Id: If4f3d6606aad40a6f1617a49681dfd45c64d3d37
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256673
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>