cleanup: delete redundant "r" open mode

Change-Id: I86ebb8c5a9dc3752e8a25f4b11b64c5be3a6429e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/390375
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

git_refs.py

@@ -105,7 +105,7 @@ class GitRefs:
     def _ReadPackedRefs(self):
         path = os.path.join(self._gitdir, "packed-refs")
         try:
-            fd = open(path, "r")
+            fd = open(path)
             mtime = os.path.getmtime(path)
         except IOError:
             return