tipc: Remove some excess struct member documentation

Remove documentation for nonexistent struct members, addressing these
warnings:

  ./net/tipc/link.c:228: warning: Excess struct member 'media_addr' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'timer' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'refcnt' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'proto_msg' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'pmsg' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'backlog_limit' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'exp_msg_count' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'reset_rcv_checkpt' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'transmitq' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'snt_nxt' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'deferred_queue' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'unacked_window' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'next_out' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'long_msg_seq_no' description in 'tipc_link'
  ./net/tipc/link.c:228: warning: Excess struct member 'bc_rcvr' description in 'tipc_link'

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>