Removal of sign-up plugin configuration related topics, associations, topic types etc

AC:

• no configuration related topics, topic types etc are left after the last migration has run
• release as 3.1
• mention the process in the release notes

The sign-up plugin's upgrade path will look as following:

• before installing sign-up 3.2 (which will not have any migration code)
• install sign-up plugin 3.1 (which is upgrade compatible with 2.x) and run once

In Sign-up 3.2 all migration will be removed.