Disable line-clamping trick in Markdown cells (#2465)

Row heights, wrapping, and Markdown-rendered cells together can
sometimes appear broken. In particular, list items whose lines are long
enough to wrap overlap the lines of list items below them.

This appears to be a problem with the height calculation for max row
height being based on a fixed line height (18px), which is not always
the case for content rendered as Markdown. It's not always easy to
reproduce, and sometimes only manifests at specific max row heights
and/or column widths, so the exact reason isn't completely clear.