Remove FOLLY_SKIP_LIBCPP_4000_THROW_BACKPORTS

Summary: 4.0 has been out for three months now, so it's pretty safe to remove them.

Reviewed By: Orvid, yfeldblum, mzlee

Differential Revision: D5096007

fbshipit-source-id: 595c3e86cd7cf1a1706f27f107fb0af8ab89a676

folly/portability/BitsFunctexcept.cpp

@@ -26,7 +26,7 @@
 
 FOLLY_NAMESPACE_STD_BEGIN
 
-#if _LIBCPP_VERSION < 4000 && !FOLLY_SKIP_LIBCPP_4000_THROW_BACKPORTS
+#if _LIBCPP_VERSION < 4000
 void __throw_length_error(char const* msg) {
   throw std::length_error(msg);
 }

folly/portability/BitsFunctexcept.h

@@ -29,7 +29,7 @@
 
 FOLLY_NAMESPACE_STD_BEGIN
 
-#if _LIBCPP_VERSION < 4000 && !FOLLY_SKIP_LIBCPP_4000_THROW_BACKPORTS
+#if _LIBCPP_VERSION < 4000
 [[noreturn]] void __throw_length_error(char const* msg); // @nolint
 [[noreturn]] void __throw_logic_error(char const* msg);
 [[noreturn]] void __throw_out_of_range(char const* msg);