Ditch line heights

Created by: MartijnCuppens

A lot of our components contain line-heights, and sm/lg versions with configurable line-heights. I wonder if it's really needed for all those components to have configurable line-heights, a mixed usage of different line heights might result in an inconsistent design, so I really doubt people really change these values per component. Also these variables look a bit strange: https://github.com/twbs/bootstrap/blob/0829decce075e190c0bb6e9992ca2a1ca56c60c7/scss/_variables.scss#L219-L220

Imo, we can just remove the different line-heights for sm/lg versions which would make our codebase a bit less bloated.

We might add some utilities to increase or decrease line heights though.

Thoughts @twbs/css-review?