Apply new printDirective changes to subgraph-js

[hwillson]

@IvanGoncharov is working on getting new printDirective changes into graphql-js (graphql/graphql-js#3362). When this is done, we're going to adjust subgraph-js to leverage these changes.

[queerviolet]

it seems like it would make sense to just remove printSubgraphSchema and direct users to this as a replacement? as of #1554, we are no longer using printSubgraphSchema in subgraph-js, though we haven't removed it, on the suspicion that users are using it.