xen-netfront: remove unused variable `extra'

This variable is supposed to hold reference to the last extra_info in the
loop. However there is only type of extra info here and the loop to process
extra info is missing, so this variable is never used and causes confusion.

Remove it at the moment. We can add it back when necessary.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>