Remove old crazy comment + fixme

Things that make you go 'WAT.' This comment was added in the patch
that split RenderBoxModelObject out of RenderBox --
eb6efbfaa79de8624ce72ad4e26ef0d3f808650e -- without any additional
explanation. I don't think it's helping anyone.

H/T to Ojan for finding today's WAT.

Review URL: https://codereview.chromium.org/718843002

git-svn-id: svn://svn.chromium.org/blink/trunk@185151 bbb929c8-8fbe-4397-9dbb-9b2b20218538