x86/paravirt: Remove set_pte_at() pv-op

On x86 set_pte_at() is now always falling back to set_pte(). So instead
of having this fallback after the paravirt maze just drop the
set_pte_at paravirt operation and let set_pte_at() use the set_pte()
function directly.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200815100641.26362-6-jgross@suse.com