Forced cleanup and removal of obsolete nodetype definitions - part two

REPO-1664 provides a first (part one) implementation of forced cleanup and removal of obsolete nodetype definitions.
The implementation in MigrateNodeTypesToV12.java however is deliberately not complete/production ready yet!
When upgrading an existing CMS v11.x repository to CMS v12.0.0 (hippo-repository 5.0.0), this class will get triggered and will perform the following tasks:

• detect migration is needed (by presence of deprecated node type hipposys:autoexport)
• ensures node type hipposys:autoexport isn't actually in use (raises a RepositoryException otherwise)
• migrate existing hippo:domain and hippo:domainrule nodetype children to become compliant with the v12.0.0 hipposys.cnd changes, REPO-1719
• migrate existing hippo:moduleconfig nodetype and /hippo:configuration/hippo:modules/eforms/hippo:moduleconfig children to become compliant with the v12.0.0 hipposys.cnd (REPO-1665) and eforms.cnd (EFORMS-477) changes
• drop the deprecated hipposys:autoexport nodetype
• and then fail with RuntimeException "Migrated to V12.0.0, please restart again."
  Note: after restart the migration no longer will be needed and only then the new V12.0.0 bootstrap initialization will be executed properly.

Clearly the last step (failing with a required restart) is not intended nor acceptable for production purposes, so as a minimum this needs to be removed before the CMS 12 GA release.
(also be noted that any upgrade also requires system parameter -Drepo.bootstrap.ignore.content.append.conflict=true to be set, only for/during the migration)

However there are several other useful/needed cleanup and removal of obsolete nodetype definitions which we can or should apply before the CMS 12 GA release, for as much feasible.

One of these will be dropping SNS and orderability from rep:root (the repository root), for which I'll create a separate subtask of this issue.
And for additional cleanup/removal steps additional subtasks can be created.

Through this issue (or yet additional/next step issues, if we want to do this more incrementally) additional enhancements/steps should be added to the MigrateNodeTypesToV12.java class, or extensions thereof.