federation: remove reference to core-js

I believe this was only needed for compatibility with old versions of
Node and hasn't been required since #311. That PR removed the explicit
dependency on core-js from federation-js/package.json, but didn't remove
this import, which breaks build scenarios that are more strict about
dependency declaration.

Fixes #692.

federation-js/src/index.ts

@@ -1,5 +1,2 @@
-import "core-js/features/array/flat";
-import "core-js/features/array/flat-map";
-
 export * from './composition';
 export * from './service';