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>