automember: Add support for action: orphans_removed

The removal of group or hostgroup orphans has been added to the automember
module.

It can be ensured that orphans have been removed using action: orphans_removed
The automember_type needs to be set for this.

New examples have been added to README-automember.md

New playbooks:
- playbooks/automember/automember-group-orphans-removed.yml
- playbooks/automember/automember-hostgroup-orphans-removed.yml

New tests:
- tests/automember/test_automember_orphans_removed.yml