mptcp: remove unused mptcp_prune_ofo_queue

The MPTCP-specific ofo-queue has been removed in mptcp_v0.93, replaced
by TCP RB-tree.

mptcp_prune_ofo_queue() was defined in mptcp_ofo_queue.c which has been
removed.

Fixes: 15d1040 ("mptcp: Build-Fix")
Fixes: 050453a ("Merge tag 'v4.9' into mptcp_trunk")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit bf3a297)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

include/net/mptcp.h

@@ -1315,9 +1315,6 @@ static inline int mptcp_subflow_count(const struct mptcp_cb *mpcb)
 u16 mptcp_select_window(struct sock *sk);
 void mptcp_tcp_set_rto(struct sock *sk);
 
-/* TCP and MPTCP flag-depending functions */
-bool mptcp_prune_ofo_queue(struct sock *sk);
-
 #else /* CONFIG_MPTCP */
 #define mptcp_debug(fmt, args...)	\
 	do {				\