net: wwan: iosm: removed unused function decl

ipc_wwan_tx_flowctrl() is declared in iosm_ipc_wwan.h but is
not defined.

Removed the dead code.
Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/wwan/iosm/iosm_ipc_wwan.h

@@ -42,14 +42,4 @@ int ipc_wwan_receive(struct iosm_wwan *ipc_wwan, struct sk_buff *skb_arg,
  *
  */
 void ipc_wwan_tx_flowctrl(struct iosm_wwan *ipc_wwan, int id, bool on);
-
-/**
- * ipc_wwan_is_tx_stopped - Checks if Tx stopped for a Interface id.
- * @ipc_wwan:	Pointer to wwan instance
- * @id:		Ipc mux channel session id
- *
- * Return: true if stopped, false otherwise
- */
-bool ipc_wwan_is_tx_stopped(struct iosm_wwan *ipc_wwan, int id);
-
 #endif