Subdiv: remove unused GPU device choice, fix crash with libepoxy on init

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 reduce 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

Differential Revision: https://developer.blender.org/D15470

release/scripts/startup/bl_ui/space_userpref.py

@@ -589,12 +589,6 @@ class USERPREF_PT_system_cycles_devices(SystemPanel, CenterAlignMixIn, Panel):
                 addon.preferences.draw_impl(col, context)
             del addon
 
-        # NOTE: Disabled for until GPU side of OpenSubdiv is brought back.
-        # system = prefs.system
-        # if hasattr(system, "opensubdiv_compute_type"):
-        #     col.label(text="OpenSubdiv compute:")
-        #     col.row().prop(system, "opensubdiv_compute_type", text="")
-
 
 class USERPREF_PT_system_os_settings(SystemPanel, CenterAlignMixIn, Panel):
     bl_label = "Operating System Settings"