Remove deprecated line-height variables

[gfellerph]

With https://github.com/swisspost/design-system/pull/1298 the named line-height variables have been deprecated. They can be removed in the next major version.

### Tasks
- [ ] Remove the variables (https://github.com/swisspost/design-system/pull/1298/files#diff-29ba5feb081f544966b5bad443a8496142bcb81a7ff12645b380b740eab45289R5)
- [ ] Update remaining usages within the Design System
- [ ] Update the documentation
- [ ] Write migration instructions
- [ ] https://github.com/swisspost/design-system/issues/3399