Revert "manifest: Explicitly install kernel-modules and /usr/sbin/iptables"
This reverts commit 85ebdb02913e59f22be9112444fcb6b0c93cff49.
This was required before because of:
https://github.com/rpm-software-management/libdnf/issues/220
But this should now be fixed in the latest stable release of rpm-ostree,
so we shouldn't need this hack anymore (this also brings back the kernel
base package).