greybus: loopback: remove unsupported version request

Remove the unsupported version request from the loopback-driver request
handler.

Unsupported requests are already handled and logged using the default
case.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>