This reverts commit 6f4e83beb0, but note:
this is not a 1:1 revert since the subscr_paging_sec_cb() semantics have changed.
In subscr_paging_dispatch(), the separate subscr parameter is needed in cases
where paging expired and there is no conn available.
For subscr_paging_sec_cb(), a conn must always be available. Hence it avoids
any duplicity by only passing the conn and deriving the subscr from that.
Callers of subscr_paging_sec_cb() pass NULL as param, so pass conn->subscr when
calling subscr_paging_dispatch() from subscr_paging_sec_cb().