Remove GdkGLProfile

Remove GdkGLProfile

The existence of OpenGL implementations that do not provide the full
core profile compatibility because of reasons beyond the technical, like
llvmpipe not implementing floating point buffers, makes the existence of
GdkGLProfile and documenting the fact that we use core profiles a bit
harder.

Since we do not have any existing profile except the default, we can
remove the GdkGLProfile and its related API from GDK and GTK+, and sweep
the whole thing under the carpet, while we wait for an extension that
lets us ask for the most compatible profile possible.

https://bugzilla.gnome.org/show_bug.cgi?id=744407