Remove unnecessary wrapper functions from ext_collections

Summary: Now that HHVM doesn't use the mangle map scheme for extensions defined in
.idl.json files, we can eliminate a bunch of unnecessary wrapper functions
in ext_collections.

Reviewed By: @jdelong

Differential Revision: D1665279

Signature: t1:1665279:1415492122:752981f56cace95bed4163612ca6710bf1fded46