repo: Remove duplicate import of 'os'

Change-Id: I9874a5deacdb6a8ce98a8a383326a5b41b1518df
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254697


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: David Pursehouse <dpursehouse@collab.net>