Remove dead code in RenderBlock::localCaretRect.

As it turns out, every RenderBlock is a RenderBlock.
So this code seems to have been dead since 2009, at least.

This also removes not one, but *two* FIXMEs.

Review URL: https://codereview.chromium.org/388353002

git-svn-id: svn://svn.chromium.org/blink/trunk@178106 bbb929c8-8fbe-4397-9dbb-9b2b20218538