Remove legacy cbang workflows

Remove old release.yml and test-build.yml workflows that referenced
'cbang' branding. These are replaced by:
- build-clang.yml: Builds null-safe clang+clangd binaries (triggered by v* tags)
- build-playground.yml: Builds WASM for playground (triggered by playground-v* tags)
- deploy-playground.yml: Deploys playground to GitHub Pages

This prevents conflicts where both old and new workflows would trigger
on v* tags.

Author: cs01