x86/mm: drop further relics of translated PV domains

x86/mm: drop further relics of translated PV domains

For PV domains paging_mode_{refcounts,translate}() are always false as
of commits 4045953527 ("x86/paging: Enforce PG_external == PG_translate
== PG_refcounts") and 92942fd3d4 ("x86/mm: drop
guest_{map,get_eff}_l1e() hooks").

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>