Delete buck2 build

Buck2 builds are published to https://github.com/facebook/buck2/releases/tag/latest,
and that is what is used by https://github.com/dtolnay/install-buck2.

.github/workflows/buck2.yml

@@ -1,18 +0,0 @@
-name: buck2
-
-on:
-  push:
-    branches:
-      - buck2
-  workflow_dispatch:
-
-jobs:
-  build:
-    uses: ./.github/workflows/build.yml
-    with:
-      crate: buck2
-      git: facebook/buck2
-      path: app/buck2
-    permissions:
-      contents: write
-    secrets: inherit