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>

net: iosm: net driver

1) Create net device & implement net operations for data/IP communication.
2) Bind IP Link to mux IP session for simultaneous IP traffic.

Signed-off-by: M Chetan Kumar <m.chetan.kumar@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>