sctp: remove an unnecessary check from sctp_endpoint_destroy

ep->base.sk gets it's value since sctp_endpoint_new, nowhere
will change it. So there's no need to check if it's null, as
it can never be null.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>