tipc: Remove callback field from subscription structure

Eliminate the "event_cb" member from TIPC's "subscription" structure
since the function pointer it holds always points to subscr_send_event().

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>