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

folly/small_vector.h

@@ -43,7 +43,6 @@
 #include <boost/mpl/size.hpp>
 #include <boost/mpl/vector.hpp>
 #include <boost/operators.hpp>
-#include <boost/type_traits.hpp>
 
 #include <folly/ConstexprMath.h>
 #include <folly/FormatTraits.h>
@@ -332,7 +331,7 @@ struct small_vector_base {
    */
   typedef typename mpl::filter_view<
       PolicyList,
-      boost::is_integral<mpl::placeholders::_1>>::type Integrals;
+      std::is_integral<mpl::placeholders::_1>>::type Integrals;
   typedef typename mpl::eval_if<
       mpl::empty<Integrals>,
       mpl::identity<std::size_t>,