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>

project.py

@@ -3227,7 +3227,7 @@ class Project:
         # Rewrite the internal state files to use relative paths between the
         # checkouts & worktrees.
         dotgit = os.path.join(self.worktree, ".git")
-        with open(dotgit, "r") as fp:
+        with open(dotgit) as fp:
             # Figure out the checkout->worktree path.
             setting = fp.read()
             assert setting.startswith("gitdir:")