[CI] Remove cache clean action

This action has never worked due to permission issues, and because
of how GitHub handles caches it wouldn't reliably help cache anyway.

.github/workflows/cache_cleanup.yml

@@ -1,32 +0,0 @@
-# https://github.com/actions/cache/blob/main/tips-and-workarounds.md#force-deletion-of-caches-overriding-default-cache-eviction-policy
-name: 🧹 Cache Cleanup
-on:
-  pull_request:
-    types:
-      - closed
-
-jobs:
-  cleanup:
-    name: Cleanup PR caches
-    runs-on: ubuntu-latest
-    permissions:
-      # `actions:write` permission is required to delete caches
-      actions: write
-      contents: read
-    steps:
-      - name: Cleanup
-        env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          GH_REPO: ${{ github.repository }}
-          BRANCH: refs/pull/${{ github.event.pull_request.number }}/merge
-        run: |
-          echo "Fetching list of cache key"
-          cache_keys_for_pr=$(gh cache list --ref $BRANCH --limit 100 --json id --jq '.[].id')
-          # Setting this to not fail the workflow while deleting cache keys.
-          set +e
-          echo "Deleting caches..."
-          for cache_key in $cache_keys_for_pr; do
-            gh cache delete $cache_key
-            echo "Deleted: $cache_key"
-          done
-          echo "Done"