Remove aliasing of glRenderbufferStorageMultisampleEXT with glRenderbufferStorageMultisample

glRenderbufferStorageMultisampleEXT is less restrictive
on multi-sample preservation and may resolve in comparison
to glRenderbufferStorageMultisample.

Hence the functionality of the latter can't be expressed
through the first.

Remove the aliasing from the gl.xml, and explicitly point
this out in the extension spec.