[llvm] [openmp] [offload][openmp] - Remove standalone build in favor of 'runtimes' (PR #170693)

[Michał Górny via llvm-commits]

mgorny wrote:

> You most likely need to set `-DOPENMP_LLVM_TOOLS_DIR=<PATH>` during the build, which is where FileCheck and lit testing tools live.

That doesn't work, and CMake indicates it isn't used:

```
CMake Warning:
  Manually-specified variables were not used by the project:

    OPENMP_LIBDIR_SUFFIX
    OPENMP_LLVM_TOOLS_DIR
    OPENMP_STANDALONE_BUILD
```

(ignore the other two, they're leftovers from old standalone build)

https://github.com/llvm/llvm-project/pull/170693