tipc: Eliminate obsolete port's "congested_link" field

Eliminate a field of the TIPC port structure that is populated,
but never referenced.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>