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

llvm-svn: 345199