x86: Move xen_post_allocator_init into xen_pagetable_setup_done

We really do not need two paravirt/x86_init_ops functions which are
called in two consecutive source lines. Move the only user of
post_allocator_init into the already existing pagetable_setup_done
function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>