Shared context support for VAOs was dropped in 0b275062

.

From the ARB_vertex_array_object spec:

   "This extension differs from GL_APPLE_vertex_array_object
   in that client memory cannot be accessed through a
   non-zero vertex array object.  It also differs in that
   vertex array objects are explicitly not sharable between
   contexts."

Nobody should be using this extension over
ARB_vertex_array_object anymore so just drop it rather than
adding locking back just for VAOs created from these
functions.

For reference the Nvidia blob doesn't expose this extension.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>