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
2 files changed