Remove the Style Compatibility Mode

[dtdesign]

The style compatibility mode was introduced with WoltLab Suite 3.1 with the intention to create an easier migration path when breaking changes occur. In reality this mode was highly intransparent in what it does and styles were pretty much never migrated, forcing us to keep those compatibility layers virtually forever.

The compatibility mode only affected some CSS changes while other changes were happily applied to styles running in older modes. To add insult to injury, templates were completely neglected, causing them to slowly drift away from the state they were when such a style was created.

The intentions of the compatibility mode were good, but the way it had been implemented is incomplete at best and has proven to cause more harm than good. It is conceptually broken and cannot be reasonably repaired, it is even questionable if this is addressable at all.