do not include api server plugin for production builds

[ryanatkn]

This changes the default config to not add the API server plugin for production builds. This may be needed for some usecases like prerendering, but for SvelteKit apps using @sveltejs/adapter-node's middleware, running the server in production doesn't work because the middleware hasn't been built yet.

Also cleans up the SvelteKit Node build a bit in the Gro adapter, removing unused files. The SvelteKit Node adapter is not configurable to choose between middleware and a standalone server, so the Gro adapter supports the deploymentMode option that defaults to middleware, signalling Gro to delete the Node adapter's index.js output and keep middlewares.js, and vice versa when server. The both option deletes nothing.