Drop simple and single+simple slot normalziation styles

Problem/Motivation

Let's drop support for simple and single+simple slot normalziation styles in 3.x (see parent for reasoning). Given that, we should simplify the code to only support the single-valued slot-normalization style and keep backwards-compatibility for the API to do that.

Note: We want to keep supporting setSlotFromCustomElement() or setSlot into a single valued CE,

API changes

Some stuff is not supported any more, so we need to write a change record.

Data model changes