remove remaining `SYNERGIA_ENABLE_CUDA/KOKKOS_ENABLE_CUDA` for portability

[s-sajid-ali]

In the interest of portability, we need to remove SYNERGIA_ENABLE_CUDA/KOKKOS_ENABLE_CUDA (which would not work for AMD/Intel GPUs). A possible solution, as mentioned by the kokkos-devs is to use:
std::is_same_v on DefaultExecutionSpace and DefaultHostExecutionSpace to detect the presence of a device backend.