ci: Stop triggering the workflow for llvm branches.

We usually do a PR for the llvm branches, so triggering a run for the branch
means we do extra work for no reason.

.github/workflows/ci.yaml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - master
-      - llvm19
 concurrency:
   # Cancels pending runs when a PR gets updated.
   group: ${{ github.head_ref || github.run_id }}-${{ github.actor }}