[NETFILTER]: nf_conntrack_expect: avoid useless list walking

Don't walk the list when unexpecting an expectation, we already
have a reference and the timer check is enough to guarantee
that it still is on the list.

This comment suggests that it was copied there by mistake from
expectation eviction:

/* choose the oldest expectation to evict */

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>