Revert "[OpenMP] Use `add_llvm_library` to build the target `PluginInterface` in `plugins-nextgen`"

Breaks cmake regeneration for me:
```
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.cuda.nextgen" which requires target "PluginInterface" that is not in any export set.
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.x86_64.nextgen" which requires target "PluginInterface" that is not in any export set.
```

This reverts commit 08c4081b.