dm vdo: Remove unused functions

get_data_vio_pool_active_discards()
get_data_vio_pool_discard_limit()
get_data_vio_pool_maximum_discards()
set_data_vio_pool_discard_limit()

are all unused since commit
a9da0fb6 ("dm vdo: remove all sysfs interfaces")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>