Removed GitHub PR auto-assign workflow

- we no longer need this because Renovate should be automerging a lot
  more, and it causes less noise for me

.github/AUTO_ASSIGN

@@ -1,2 +0,0 @@
-renovate/tryghost-kg-.* kevinansfield
-renovate/.* daniellockyer

.github/workflows/auto-assign.yml

@@ -1,15 +0,0 @@
-name: Auto Assign
-on:
-  pull_request:
-    types: [opened, reopened]
-jobs:
-  automate:
-    runs-on: ubuntu-22.04
-    if: github.repository_owner == 'TryGhost'
-    env:
-      FORCE_COLOR: 1
-      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-    steps:
-      - uses: actions/checkout@v4
-      - run: gh extension install daniellockyer/gh-auto-assign
-      - run: gh auto-assign