Drop the rd.cos.X parameters on 3.0

[Itxaka]

Currently we support both the old rd.cos.X and the newer rd.immucore.X parameters in the cmdline to configure several things under the immutable module (see more on https://github.com/kairos-io/immucore#kernel-configuration-parameters)

We should drop those under 3.0 as the newer ones should be used. Would drop a bit of code as well as a side effect.

[mauromorales]

Sounds good to me. Maybe we can already have a card to show a deprecation notice.