Remove incorrect RenderTableCell FIXME

We should not implement this change, since table cells would not update if we
later set their row/colspan to 1.

Review URL: https://codereview.chromium.org/688683004

git-svn-id: svn://svn.chromium.org/blink/trunk@185183 bbb929c8-8fbe-4397-9dbb-9b2b20218538