Prior to SERVER-63816, we needed to reset the _docsReturnedByIdLookup metric, in case the $search is part of a $lookup subpipeline since the subpipeline was parsed multiple times. Now the subpipeline is only parsed once, so we don't need to reset that metric. We can remove the function resetDocsReturnedByIdLookup() from SharedSearchState.

While we're at it, we shouldn't have two separate constructors for 
DocumentSourceInternalSearchIdLookUp. Instead, we should have one constructor where the limit parameter is set with default value 0.