Revert "Teach __libcpp_is_floating_point that __fp16 and _Float16 are"

This reverts commits r333103 and r333108. _Float16 and __fp16 are C11
extensions and compilers other than Clang don't define these for C++.

Differential Revision: https://reviews.llvm.org/D53670

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345199 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 842383f)

rdar://47617754

Author: petrhosek

include/type_traits

@@ -733,12 +733,6 @@ _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_integral_v
 // is_floating_point
 
 template <class _Tp> struct __libcpp_is_floating_point              : public false_type {};
-#ifdef __clang__
-template <>          struct __libcpp_is_floating_point<__fp16>      : public true_type {};
-#endif
-#ifdef __FLT16_MANT_DIG__
-template <>          struct __libcpp_is_floating_point<_Float16>    : public true_type {};
-#endif
 template <>          struct __libcpp_is_floating_point<float>       : public true_type {};
 template <>          struct __libcpp_is_floating_point<double>      : public true_type {};
 template <>          struct __libcpp_is_floating_point<long double> : public true_type {};