Remove type defs for non-ts files and fix wrong import for router in _app (#7150)

packages/next-server/dynamic.d.ts

-export * from './dist/lib/dynamic'
-export {default} from './dist/lib/dynamic'

packages/next/dynamic.d.ts

-export * from 'next-server/dynamic'
-export {default} from 'next-server/dynamic'

packages/next/link.d.ts

-export * from './dist/client/link'
-export {default} from './dist/client/link'

packages/next/pages/_app.tsx

@@ -5,7 +5,7 @@ import { makePublicRouterInstance } from '../client/router'
 
 export { NextComponentType, IContext, IAppContext, IAppInitialProps, IAppProps }
 
-type Router = import('next-server/lib/router/router').default
+type Router = import('next-server/dist/lib/router/router').default
 
 export type AppClientContext = IAppContext<Router>