remove ip_forward warning

Now that docker sets /proc/sys/net/ipv4/ip_forward by default (unless
the user manually specifies "-ip-forward=false"), there's no need to
warn if its disabled.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3811 from jpoimboe/remove-ip_forward-warning

remove ip_forward warning