Remove checks for out of flow RenderFlowThreads.

I believe this is no longer possible now that we only
use Regions for multi-column.

Review URL: https://codereview.chromium.org/297683011

git-svn-id: svn://svn.chromium.org/blink/trunk@174744 bbb929c8-8fbe-4397-9dbb-9b2b20218538