Simplify Schema for Categorised and Graduated Symbology

[arjxn-py]

I'm wondering how much we could simplify the schema itself to only have color steps and radius steps, not dealing with open-layers implementation details about circle-fill-color and fill-color at all in the schema.

Originally posted by @martinRenou in #697 (comment)

[martinRenou]

I won the race #698 🥇