plugins: remove additive context flavours

[KnorpelSenf]

Previously, we only had additive context flavours that worked via type intersection.

Later, we added transformative context flavours that are strictly more powerful. All additive flavour can be expressed that way.

We should rework all plugin to only use transformative flavours in order to avoid confusion. People find it hard to understand how to combine the two types of types.

This should be dropped from the docs, as well.