repo: fix unused variable usage

The refactoring here left behind a variable reference that no
longer exists.  Clean it up.

Bug: https://crbug.com/gerrit/11144
Change-Id: Ifdb7918b37864c48f3deef27c8bae3f793275d35
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/236613
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

repo

@@ -443,7 +443,7 @@ def _CheckGitVersion():
     raise CloneFailure()
 
   if ver_act is None:
-    print('error: "%s" unsupported' % ver_str, file=sys.stderr)
+    print('fatal: unable to detect git version', file=sys.stderr)
     raise CloneFailure()
 
   if ver_act < MIN_GIT_VERSION: