github: remove nix-update-flake action

Summary: It needs new credentials to act as a robot account on my behalf, rather
than the 'github-actions' user, as that one can't be approved for security/legal
reasons from Google.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: I72e303b5625a6417d3444fc65cb79a779a5d9d53

.github/workflows/nix-update-flake.yml

@@ -1,22 +0,0 @@
-name: Update nix flake.lock
-on:
-  workflow_dispatch: # allows manual triggering
-  schedule:
-    - cron: '40 3 * * 0' # runs weekly on Sunday at 03:40
-
-jobs:
-  lockfile:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
-      - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@cd46bde16ab981b0a7b2dce0574509104543276e
-      - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@da2fd6f2563fe3e4f2af8be73b864088564e263d
-        with:
-          pr-title: "nix: update flake.lock"
-          pr-assignees: thoughtpolice
-          pr-reviewers: thoughtpolice
-          pr-labels: |
-            dependencies