[CUDA] Remove implementations of nexttoward.

Summary:
__builtin_nexttoward lowers to a libcall, e.g. nexttowardf(), that CUDA
does not have.

Rather than try to implement it, we simply remove these functions --
nvcc doesn't support them either, and nextafter, which does work, does
essentially the same thing on GPUs, because GPUs don't have long double.

Reviewers: tra

Subscribers: cfe-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D40152

llvm-svn: 318494