[UX] Remove the textarea_with_summary field type

In 80% of use cases, truncating a field to a specified number of characters makes sense for teaser display modes. For those few use-cases where you actually want different content, what you really should be using is a different field.

Having these two concepts contained in a single field is confusing for users, and will become even more confusing when we introduce a Rich-text editor.

We may need to include some kind of an upgrade path for sites that were using the splitter before, but I think that could be done reasonably easily.