usb: remove not used variable in usb_ether_curr_dev

grepping for usb_ether_curr_dev in u-boot source code shows

$ grep -r usb_ether_curr_dev .
./cmd/usb.c:static int __maybe_unused usb_ether_curr_dev = -1; /* current ethernet device */
$

only declared but never used, so it can safely
removed from code.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>

cmd/usb.c

@@ -26,9 +26,6 @@
 #ifdef CONFIG_USB_STORAGE
 static int usb_stor_curr_dev = -1; /* current device */
 #endif
-#if defined(CONFIG_USB_HOST_ETHER) && !defined(CONFIG_DM_ETH)
-static int __maybe_unused usb_ether_curr_dev = -1; /* current ethernet device */
-#endif
 
 /* some display routines (info command) */
 static char *usb_get_class_desc(unsigned char dclass)