Latest OpenSubdiv builds without GLEW by default, which is also what we do
for precompiled libraries. So there is no need for compatibility checking
with system GLEW.

Additionally WITH_SYSTEM_GLEW is turned off by default for Blender, and this
logic was presumably added when it was still on by default a few years ago.

Also remove outdated mention of glew-mx, we use intern/glew-mx and no
external library for this.