[libomptarget] Disable build of amdgpu plugin as it doesn't build with rocm.

Author: JonChesterfield

openmp/libomptarget/plugins/CMakeLists.txt

@@ -66,7 +66,6 @@ endif()
 endmacro()
 
 add_subdirectory(aarch64)
-add_subdirectory(amdgpu)
 add_subdirectory(cuda)
 add_subdirectory(ppc64)
 add_subdirectory(ppc64le)