After SERVER-103991 Express path can avoid fetching the document if the projection is covered by an index.

However, the projection would still be here: 

https://github.com/10gen/mongo/blob/28e902156350243be96797a18b3ad9c25e292ca8/src/mongo/db/exec/express/express_plan.h#L1359

This doesn't affect correctness, but can speed up express path even more.

We should find a way to communicate if projection is covered to avoid this duplication.