openSubdiv_init() would detect available evaluators before any OpenGL context
exists, causing a crash with libepoxy. This test however is redundant as we
already check the requirements on the Blender side through the GPU API.

To simplify things, completely remove the device detection in the opensubdiv
module and reduces the evaluators to just CPU and GPU. The plan here is to move
to the GPU module abstraction over OpenGL/Metal/Vulkan and so all these
different backends no longer make sense.

This also removes the user preference for OpenSubdiv compute device, which was
not used for the new GPU subdivision implementation.

Ref D15291