mptcp: Remove unused declaration mptcp_sockopt_sync()

Commit a1ab24e5 ("mptcp: consolidate sockopt synchronization")
removed the implementation but leave declaration.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240816100404.879598-1-yuehaibing@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.h

@@ -1154,7 +1154,6 @@ static inline void mptcp_pm_close_subflow(struct mptcp_sock *msk)
 	spin_unlock_bh(&msk->pm.lock);
 }
 
-void mptcp_sockopt_sync(struct mptcp_sock *msk, struct sock *ssk);
 void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk);
 
 static inline struct mptcp_ext *mptcp_get_ext(const struct sk_buff *skb)