refactor(code-runtime): remove subprocess backend

scripts/check-workspace-constraints.ts

@@ -148,7 +148,6 @@ function expectedDshPackageFiles(manifest: PackageManifest): readonly string[] {
     'lib/invariant.js',
     ...manifest.bin ? ['lib/bin.js'] : [],
     ...manifest.exports?.['./worker'] ? ['lib/worker.cjs'] : [],
-    ...exportDefault(manifest, './runtime-host') === './lib/runtime-host.js' ? ['lib/runtime-host.js'] : [],
     // UI plugin packages ship their browser bundle beside the node lib
     // (single-artifact ruling: dist/ retired, ./client resolves lib/client.js).
     // Keyed on the artifact path, not the subpath name: apiproxy's ./client is