x86/mm: no-one passes a NULL domain to init_xen_l4_slots()

x86/mm: no-one passes a NULL domain to init_xen_l4_slots()

Drop the NULL checks - they've been introduced by commit 8d7b633ada
("x86/mm: Consolidate all Xen L4 slot writing into
init_xen_l4_slots()") without giving a reason; I'm told this was done
in anticipation of the function potentially getting called with a NULL
argument down the road.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>