folly/fibers/Traits: remove dependency on boost/type_traits
folly/fibers/Traits: remove dependency on boost/type_traits

Summary: Small change so that a template class `ExtractFirstArg` can do the same thing without relying on an expensive header, `boost/type_traits.hpp`.

Reviewed By: yfeldblum

Differential Revision: D7781675

fbshipit-source-id: 1e5f431217da6cdd31ba1d77a8fc2b19a43808aa

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly