CXX-1539 remove using from collection header
CXX-1539 remove using from collection header

acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver