fix: remove manual external override to fix workspace dependency resolution

The root cause was manually overriding rollupConfig.external which broke
nx's automatic workspace dependency resolution mechanism. By removing this
override and letting nx handle externals via project.json, TypeScript can
now properly resolve workspace dependencies during the rollup build.

This aligns rsbuild-plugin with working packages like cli and data-prefetch
that don't manually override external configuration.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ScriptedAlchemy