Cut legacy use of is_reflectable_* in reflection/serializer.h
Cut legacy use of is_reflectable_* in reflection/serializer.h

Summary:
[Thrift] Cut legacy use of `is_reflectable_*` in `reflection/serializer.h`.

It is currently the case that the reflection generated headers for a given module include the corresponding reflection generated headers from all included modules (not exactly but close enough for this summary). This was not always the case. This use of `is_reflectable_*` corresponds to that previous time.

Reviewed By: vitaut

Differential Revision: D9625022

fbshipit-source-id: cbe9a9a9d6b870abfaa04d07cc71de4bb68156ea

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