drivers: xen: gnttab: remove redundant GNTTABOP_setup_table call

Initially this driver was a port from mini-os. Michal Orzel
(@orzelmichal) pointed that it contains incorrect grant table
initialization sequence and redundant calls. Driver mapped grant table
frames via loop of XENMEM_add_to_physmap calls and then tried to do the
same but via GNTTABOP_setup_table operation. After completion of latter
it did not even use provided frames list. This did not cause any major
issues, since XENMEM_add_to_physmap correctly map gnttab frames that
were used.

Remove redundant GNTTABOP_setup_table call from grant table driver to
clean up its initialization sequence.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>