Bug 1580458 - remove wchar.h include from TypeTraits.h; r=dmajor

The only plausible reason I can think of to include this header here is
for `wchar_t`, but `wchar_t` is a builtin type.  So let's get rid of the
header.

Differential Revision: https://phabricator.services.mozilla.com/D45514

--HG--
extra : moz-landing-system : lando

mfbt/TypeTraits.h

@@ -17,8 +17,6 @@
  * platforms provide that header, we can convert all users and remove this one.
  */
 
-#include <wchar.h>
-
 namespace mozilla {
 
 /* Forward declarations. */