Merges all of the styles in the Zebra module into the default styles.
- skins.vector.zebra.styles ResourceLoader module is removed.
- FIXME's related to Zebra styles are removed.
- Top-level "&" that were used for scoping to feature-flag are removed.
- `$wgVectorZebraDesign` feature flag is removed.
Bug: T347712
Change-Id: I0177541ae87a5d070d5317629ca38de93613d620