net: caif: Remove unused declaration cfsrvl_ctrlcmd()

Commit 43e36921

 ("caif: Move refcount from service layer to sock and dev.")
declared but never implemented this.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230809134943.37844-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>