sctp: Remove unused function declarations

These declarations are never implemented since beginning of git history.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20230731141030.32772-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>