drivers: net: xen-netfront: Fixed W=1 set but unused warnings

drivers/net/xen-netfront.c:2416:16: warning: variable ‘target’ set but not used [-Wunused-but-set-variable]
 2416 |  unsigned long target;

Remove target and just discard the return value from simple_strtoul().

This patch does give a checkpatch warning, but the warning was there
before anyway, as this file has lots of checkpatch warnings.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201031180435.1081127-1-andrew@lunn.chSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/xen-netfront.c

@@ -2413,12 +2413,11 @@ static ssize_t store_rxbuf(struct device *dev,
 			   const char *buf, size_t len)
 {
 	char *endp;
-	unsigned long target;
 
 	if (!capable(CAP_NET_ADMIN))
 		return -EPERM;
 
-	target = simple_strtoul(buf, &endp, 0);
+	simple_strtoul(buf, &endp, 0);
 	if (endp == buf)
 		return -EBADMSG;