[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