README: Remove outdated information regarding the ifcfg-rh plugin.
README: Remove outdated information regarding the ifcfg-rh plugin.

The storage plugin used by NM is an implementation detail that the role
does not care about, since it uses NM via libnm.

Signed-off-by: Till Maas <279a4cf6d7f8da1ddc113bb946b9273d9d938683@till.name>

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network