SW: drop rng-tools from GRMLBASE

This was noticed via a dependency on mksh (rng-tools, being a
transitional dummy package nowadays, actually Pre-Depends on mksh).
This saves us ~1,671 kB and we already ship haveged. If we really want
to provide such a feature (to feature an entropy gathering daemon to
support hardware randon number generators), then we should probably look
into rng-tools5 instead.