Simplified handling of GPU device IDs

Simplified handling of GPU device IDs

Removed gmx_gpu_device_id lookup function, whose indirect lookup of
the device ID was more complicated than we need. We already require
the user to select GPUs based on the IDs as understood by CUDA, so for
simplicity, and without loss of generality, we can do that everywhere
in the code.

Removed a previously unused function for querying the name of an
OpenCL device.

Change-Id: I6557dd51f6b23591d4fd2b6383deb6abf58a0b92