[llvm] [Offload] Remove unnecessary omp CMake target dependencies (PR #149060)

[Kenneth Benzie via llvm-commits]

kbenzie wrote:

> > IIRC there are indeed some function calls from `libomptarget` to `libomp`.
> 
> Yeah, there are a few but I forget which ones. Did this really build and pass the tests?

The build works with and without `openmp` in `LLVM_ENABLE_RUNTIMES`. I'll admit I don't have a fully functional OpenMP test environment set up currently so I'm not sure about that.

Would `libomptarget` be dynamically loading symbols from `libomp` at all?
If not, I'm not sure I understand how the build can be successful if there are actually uses of symbols from that library.

https://github.com/llvm/llvm-project/pull/149060