dccp: remove unused inline function dccp_set_seqno

There's no callers in-tree since commit 792b4878 ("dccp: Implement
both feature-local and feature-remote Sequence Window feature")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>