[OpenMP] Dropped unnecessary define when compiling deviceRTLs for NVPTX

The comment said CUDA 9 header files use the `nv_weak` attribute which
`clang` is not yet prepared to handle. It's three years ago and now things have
changed. Based on my test, removing the definition doesn't have any problem on
my machine with CUDA 11.1 installed.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D94700