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.com


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