lightnvm: pblk: remove unused smeta_ssec field

smeta_ssec field in pblk_line is never used after it was replaced by
the function pblk_line_smeta_start().

Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>