Remove main to master mirroring. (#2163)

All the workflows have been migrated to use main and we are archiving
the master branch.

.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 packages 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/packages' }}
-    name: Mirror main branch to master branch
-    steps:
-      - name: Mirror action step
-        id: mirror
-        uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
-        with:
-          github-token: ${{ secrets.FLUTTERMIRRORINGBOT_TOKEN }}
-          source: 'main'
-          dest: 'master'