folly/String: drop unneeded type_traits header
folly/String: drop unneeded type_traits header

Summary: Unneeded header in frequently-used file; drop it

Reviewed By: yfeldblum

Differential Revision: D7165040

fbshipit-source-id: b60a3cdf924ab93961a7ead3ac0653a2339a4faf

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