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>

repo

@@ -91,7 +91,6 @@ if __name__ == '__main__':
 
 # repo default configuration
 #
-import os
 REPO_URL = os.environ.get('REPO_URL', None)
 if not REPO_URL:
   REPO_URL = 'https://gerrit.googlesource.com/git-repo'