Revert "CluesToConcepts: Get concepts even for the short ClueTokens"

This reverts commit 01718ca36a80a8f4708a83127affe4cd7182d856.
This meant a massive deterioration of performance; linking concepts
to the question is imho still a good idea, but it will need much
more careful execution, first we need to tweak fulltext search query
implications etc.