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

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.ch
Signed-off-by: Jakub Kicinski <kuba@kernel.org>