First pass at removing dead vertical writing mode code.

This just removes a random subset of vertical writing mode bits
that I grepped for. There's a ton more to do, but it seems best to
do it in chunks.

The key things for understanding this patch, isWritingModeRoot is
always false and isHorizontalWritingMode is always true. Also,
we're never flipped* modes of any kind, so we can undo any flipping.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/688213002