ci: remove gitlab mirror workflow

.github/workflows/mirror-to-gitlab.yml

@@ -1,33 +0,0 @@
-name: Mirror to GitLab
-
-on:
-  push:
-    branches: ['**']
-    tags: ['**']
-  delete:
-  workflow_dispatch:
-
-concurrency:
-  group: mirror-to-gitlab
-  cancel-in-progress: false
-
-jobs:
-  mirror:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout full history
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
-
-      - name: Setup SSH
-        run: |
-          mkdir -p ~/.ssh
-          echo "${{ secrets.GITLAB_SSH_PRIVATE_KEY }}" > ~/.ssh/id_ed25519
-          chmod 600 ~/.ssh/id_ed25519
-          ssh-keyscan -t rsa,ecdsa,ed25519 gitlab.com >> ~/.ssh/known_hosts
-
-      - name: Push to GitLab
-        run: |
-          git remote add gitlab "${{ secrets.GITLAB_REPO_URL }}"
-          git push --mirror gitlab