docs: remove ip_forward enabling step

Now that docker sets /proc/sys/net/ipv4/ip_forward by default, remove
the step to enable it in the installation docs.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3813 from jpoimboe/update-ip_forward-docs

docs: remove ip_forward enabling step