xen-netfront: remove useless NULL check in netif_free

xn_ifp is allocated in create_netdev with if_alloc(IFT_ETHER).
According to the current arrangement it can't be NULL.

Coverity ID: 1349805
Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D5252