usb: gadget: config: Remove unused usb_gadget_config_buf

usb_gadget_config_buf() has been unused since 2012's
commit fa06920a ("usb: gadget: Remove File-backed Storage Gadget
(g_file_storage).")

Remove it.

Signed-off-by: "Dr. David Alan Gilbert" <linux@treblig.org>
Link: https://lore.kernel.org/r/20250608233338.179894-2-linux@treblig.org


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