[Clang] Remove `CLANG_OPENMP_NVPTX_DEFAULT_ARCH` CMake option.

[Clang] Remove `CLANG_OPENMP_NVPTX_DEFAULT_ARCH` CMake option.

The `CLANG_OPENMP_NVPTX_DEFAULT_ARCH` is a static build configuration to
set the default OpenMP value. This was replaced in D141708 with the use
of the `nvptx-arch` tool which lets us query this at runtime instead.
This makes the behaviour between AMD and NVIDIA be more consisten and
allows users to have a default architecture that does not rely on
whoever configured the LLVM build.

Depends on D141708

Reviewed By: tra, MaskRay

Differential Revision: https://reviews.llvm.org/D141723