Removing dtype check in hip backend

[benvanwerkhoven]

As @stijnh suggested in #280, it might be possible to remove the dtype check that is currently performed by the HIP backend on this line:
https://github.com/KernelTuner/kernel_tuner/blob/master/kernel_tuner/backends/hip.py#L111

As we only need the number of bytes in Python, the pointer that is passed to the GPU kernel could just be a void pointer.

[fjwillemsen]

Verdict: this is inconsequential, if dtype_str in dtype_map.keys(): can be removed.