replace legacy g_multi usb gadget module with configfs setup

If only to get rid of unneeded, legacy kernel code, we could setup the gadget devices we need via configfs (see https://gitlab.com/postmarketOS/pmaports/-/blob/master/main/postmarketos-mkinitfs/init_functions.sh#L281 for show postmarketos does it, as an example) instead of including g_multi (and other legacy gadget modules).