meta: remove update-flake-lock GHA job

.github/workflows/update-flake-lock.yaml

@@ -1,22 +0,0 @@
-name: Update flake.lock
-on:
-  workflow_dispatch: # allows manual triggering
-  schedule:
-    - cron: '0 0 * * 0'
-
-jobs:
-  lockfile:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v2
-      - name: Install Nix
-        uses: cachix/install-nix-action@v16
-      - name: Get current date
-        id: date
-        run: echo "::set-output name=date::$(date +'%Y%m%d')"
-      - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v13
-        with:
-          pr-title: "Updates ${{ steps.date.outputs.date }}"
-          branch: "updates-${{ steps.date.outputs.date }}"