x86/xen: drop tests for highmem in pv code

With support for 32-bit pv guests gone pure pv-code no longer needs to
test for highmem. Dropping those tests removes the need for flushing
in some places.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>