tipc: remove tipc_core_start/stop routines

Remove redundant wrapper functions like tipc_core_start() and
tipc_core_stop(), and directly move them to their callers, such
as tipc_init() and tipc_exit(), having us clearly know what are
really done in both initialization and deinitialzation functions.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Tested-by: Tero Aho <Tero.Aho@coriant.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>