Drop pin_keys (originally introduced to support v2 to v3 migration)

Remove obsolete code, which looked for subscriptions without an associated key, and tried to find a distinctly matching key. Originally this was implemented to help with a shortcoming of code which handled v2 to v3 migration. As the support to migrate v2 lists to v3 is dropped in v4, remove the pinning code as well.

Closes #411 (closed)