Clean up MethodMetadata
Clean up MethodMetadata

Summary: Removes unused bits and outdated comments, and replaces OwningTag with in-place constructor to reduce boilerplate

Reviewed By: thedavekwon

Differential Revision: D35081994

fbshipit-source-id: 6cd3bb9fda89824bf0a51f1bfa9032f2483e2575

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift