[libomptarget] Always enable time tracing in libomptarget

Previously time tracing features were hidden behind an optional CMake
option. This was because `libomptarget` was not based on the LLVM
libraries at that time. Now that `libomptarget` is an LLVM library we
should be able to freely use the `LLVMSupport` library whenever we want
and do not need to guard it in this way.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D132852