[IPV4]: Fix issue reported by Coverity in ipv4/fib_frontend.c

fib_del_ifaddr() dereferences ifa->ifa_dev, so the code already assumes that
ifa->ifa_dev is non-NULL, the check is unnecessary.

Signed-off-by: Jayachandran C. <c.jayachandran at gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>