Remove dual-type support for Languages and Categories

The new form was implemented in Parsoid and deployed in October, so enough time
has passed that there is no longer any need for VisualEditor to support the old
type.

Bug: 56009
Change-Id: Ief2e584345ccdb0e7eb44ab16c4bc14f7c14f5ec
Merge "Remove dual-type support for Languages and Categories"