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

This is still not working for me:
```
-- Configuring done
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.amdgpu" which requires target "elf_common" that is not in any export set.
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.cuda" which requires target "elf_common" that is not in any export set.
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.x86_64" which requires target "elf_common" that is not in any export set.
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.cuda.nextgen" which requires target "elf_common" that is not in any export set.
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 "elf_common" 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.
-- Generating done
```

This reverts commit e682a76c.