ci: remove pr-size-labeler backfill workflow (#1103)

The action refuses to run unless GITHUB_EVENT_NAME=="pull_request",
so workflow_dispatch no-ops. Backfill was completed out-of-band via
a local script calling the REST labels endpoint; this one-off
workflow is no longer useful and is being deleted.

Co-authored-by: Orca <help@stably.ai>

.github/workflows/pr-size-labeler-backfill.yml

@@ -1,47 +0,0 @@
-# One-off workflow to backfill size labels on all open PRs.
-# Runs cbrgm/pr-size-labeler-action against each open PR number via matrix.
-# Delete this file after the backfill completes.
-
-name: PR Size Labeler (backfill)
-
-on:
-  workflow_dispatch:
-
-permissions:
-  pull-requests: write
-  contents: read
-
-jobs:
-  enumerate:
-    runs-on: ubuntu-latest
-    outputs:
-      prs: ${{ steps.list.outputs.prs }}
-    steps:
-      - name: List open PR numbers
-        id: list
-        env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        run: |
-          prs=$(gh pr list --repo "${{ github.repository }}" --state open --limit 500 --json number --jq '[.[].number]')
-          echo "Found PRs: $prs"
-          echo "prs=$prs" >> "$GITHUB_OUTPUT"
-
-  label:
-    needs: enumerate
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      max-parallel: 5
-      matrix:
-        pr: ${{ fromJSON(needs.enumerate.outputs.prs) }}
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-
-      - name: Label PR ${{ matrix.pr }} based on size
-        uses: cbrgm/pr-size-labeler-action@v1.3.7
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
-          github_repository: ${{ github.repository }}
-          github_pr_number: ${{ matrix.pr }}
-          config_file_path: .github/pull-request-size.yml