[New Multicolumn] Remove RenderRegionSet.

It only did one thing, and that one thing can be done directly by
RenderMultiColumnSet instead.

Review URL: https://codereview.chromium.org/267903003

git-svn-id: svn://svn.chromium.org/blink/trunk@173411 bbb929c8-8fbe-4397-9dbb-9b2b20218538