net: remove duplicate definition of ETHADDR_WILDCARD

ETHADDR_WILDCARD is defined as the same value in both env_flags.h
and env_callback.h

As env_callback.h includes env_flags.h, remove the duplicate definition
from env_callback.h

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>