ci: remove flake-lock-update workflow for now

.forgejo/workflows/flake-lock-update.yml

@@ -1,19 +0,0 @@
-on: [push]
-jobs:
-  check:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v3
-      - name: install nix action
-        uses: https://github.com/DeterminateSystems/nix-installer-action@main
-        with:
-          github-token: ${{ secrets.GH_TOKEN }}
-      - name: update flake.lock
-        run: nix flake update
-      - name: create PR
-        uses: https://github.com/peter-evans/create-pull-request@v5
-        with:
-          token: ${{ secrets.UPDATE_TOKEN }}
-          committer: notohh <github@notohh.dev>
-          author: forgejo <dev@notohh.dev
-          branch: flake-lock-update