Remove mirroring workflow. (#1822)

The master branch has been successfully migrated to main and main has
been used as the default branch for a few months already.

Bug: https://github.com/flutter/flutter/issues/90476

## Pre-launch Checklist

- [X] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [X] I signed the [CLA].
- [X] I read the [Contributors Guide].
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

.github/workflows/mirror.yml

@@ -1,28 +0,0 @@
-# Copyright 2013 The Flutter Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-# Mirror master to main branches in the gallery repository.
-on:
-  push:
-    branches:
-      - 'main'
-
-# Declare default permissions as read only.
-permissions: read-all
-
-jobs:
-  mirror_job:
-    permissions:
-      pull-requests: write
-    runs-on: ubuntu-latest
-    if: ${{ github.repository == 'flutter/samples' }}
-    name: Mirror main branch to master branch
-    steps:
-      - name: Mirror action step
-        id: mirror
-        uses: google/mirror-branch-action@30c52ee21f5d3bd7fb28b95501c11aae7f17eebb
-        with:
-          github-token: ${{ secrets.FLUTTERMIRRORINGBOT_TOKEN }}
-          source: 'main'
-          dest: 'master'