staging: r8188eu: remove unused function prototype

There's no rtw_write_scsi function in this driver. Remove its prototype.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210908085747.32254-2-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>