Migrate remaining folly::void_t callers to std::void_t

Summary:
Replace `folly::void_t` with `std::void_t` in the remaining leaf callers
outside of `folly` itself, and drop `fbsystrace`'s private verbatim copy of
folly's `void_t` in favor of the standard one.

See the base of this stack for why `std::void_t` is a behavior-preserving
replacement: the [CWG 1558](http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558)
defect motivating folly's implementation has been resolved and is implemented
correctly by GCC, Clang, and MSVC.

`FBIsNullaryCallableButNotObjCBlock.h` only mentioned `folly::void_t` in a
comment; the reference is updated for accuracy.

`#include <folly/Traits.h>` is dropped from `RouteHandleTestUtil.h` and
`TypedIdentifierTest.cpp`, which no longer use anything from it.

Differential Revision: D115143902

fbshipit-source-id: 8c29c3895a60cc8b75332624411a8297ca075e78

Author: r-barnes

mcrouter/lib/test/RouteHandleTestUtil.h

@@ -9,10 +9,10 @@
 
 #include <memory>
 #include <string>
+#include <type_traits>
 #include <vector>
 
 #include <folly/Optional.h>
-#include <folly/Traits.h>
 #include <folly/fibers/FiberManager.h>
 #include <folly/fibers/SimpleLoopController.h>
 #include <folly/fibers/WhenN.h>
@@ -346,7 +346,7 @@ struct RecordingRoute {
   template <typename T>
   struct has_app_error<
       T,
-      folly::void_t<decltype(std::declval<T>().appSpecificErrorCode_ref())>>
+      std::void_t<decltype(std::declval<T>().appSpecificErrorCode_ref())>>
       : std::true_type {};
 
   template <
@@ -364,9 +364,7 @@ struct RecordingRoute {
   template <typename T, typename = void>
   struct has_exptime : std::false_type {};
   template <typename T>
-  struct has_exptime<
-      T,
-      folly::void_t<decltype(std::declval<T>().exptime_ref())>>
+  struct has_exptime<T, std::void_t<decltype(std::declval<T>().exptime_ref())>>
       : std::true_type {};
 
   template <