Delete User-Agent updater action

Since it's updated with TDesktop sources, it's mostly useless for me.

.github/workflows/user_agent_updater.yml

@@ -1,18 +0,0 @@
-name: User-agent updater.
-
-on:
-  repository_dispatch:
-    types: ["Restart user_agent_updater workflow."]
-  schedule:
-    # At 00:00 on day-of-month 1.
-    - cron: "0 0 1 * *"
-  pull_request_target:
-    types: [closed]
-
-jobs:
-  User-agent:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: desktop-app/action_code_updater@master
-        with:
-          type: "user-agent"