platform/x86: intel_scu_ipc: Drop unused prototype intel_scu_ipc_fw_update()

There is no implementation for that anymore so drop the prototype.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>