libceph: redo ceph_calc_pg_primary() in terms of ceph_calc_pg_acting()

Reimplement ceph_calc_pg_primary() in terms of ceph_calc_pg_acting()
and get rid of the now unused calc_pg_raw().

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>