chore(ci): remove upstream branch-checks.yml with NVIDIA self-hosted runners

The branch-checks.yml workflow was inherited from NVIDIA/OpenShell and
uses self-hosted runner labels (build-amd64, build-arm64) and a private
NVIDIA container image (ghcr.io/nvidia/openshell/ci:latest) that do not
exist in the kagenti fork. All 6 jobs queue indefinitely waiting for
runners that will never appear.

The equivalent checks (Rust, Python, Markdown) are already covered by
kagenti-ci.yml using ubuntu-latest runners and pass correctly.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Paolo Dettori <dettori@us.ibm.com>

Author: pdettori