Cut extra return in Core::doCallback

Summary: [Folly] Cut extra `return` statement in `Core<T>::doCallback`.

Reviewed By: Orvid

Differential Revision: D15527963

fbshipit-source-id: 9396f9ac2b246676b33d13c87e24dd42198a11ec

folly/futures/detail/Core.h

@@ -650,7 +650,6 @@ class Core final {
       };
       RequestContextScopeGuard rctx(std::move(context_));
       callback_(executor_.copy(), std::move(result_));
-      return;
     }
   }