Drop .pre-scrollable class

[MartijnCuppens]

So that #28917 can be merged. This class is probably not used that much, we don't even use it in our docs ourselves. Custom css can be written to replace this class.

TODO:

• Document in migration

[vanillajonathan]

For anyone who relied on this class, here is the CSS code previously used.

.pre-scrollable {
  max-height: 340px;
  overflow-y: scroll;
}