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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Is #136729 related to that?
> 
> I'm wondering if it would it be possible to move the implementations of these libomp entry points into libomptarget and have libomp be implemented in terms of libomptarget rather than this circular dependency situation?

Yes, it's related but only moves out the OpenMP device library. What we should do is re-write `libomptarget`'s plugin layer to use the newly created API and then move all the code into `openmp`. Then we just link the offloading runtime when we build.

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