repl: Rely on block decorations to size according to content (#15713)

We no longer have to use line height calculations now that
https://github.com/zed-industries/zed/pull/15536 is in. Blocks resize
according to their content. This fixes up some of the rendering issues
on plaintext outputs as well.

Release Notes:

- Fix repl plain text output wrapping around and covering editor text
(https://github.com/zed-industries/zed/issues/15491,
https://github.com/zed-industries/zed/issues/14855)