can: usb_8dev: remove unused member echo_skb from struct usb_8dev_priv

This patch removes the unused memberecho_skb from the struct
usb_8dev_priv.

Fixes: 0024d8ad ("can: usb_8dev: Add support for USB2CAN interface from 8 devices")
Link: https://lore.kernel.org/all/20220104230753.956520-1-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>