folly/small_vector: remove unneeded include of `type_traits.hpp`
folly/small_vector: remove unneeded include of `type_traits.hpp`

Summary: Included but not used.  Dropped this (expensive) include

Reviewed By: Orvid

Differential Revision: D7781674

fbshipit-source-id: 11180d34cda63ef91f70c64c84bf89346e5d1729

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