Smart group cache isn't rebuilt when a group is no longer a parent group

A trivial bug, and one I won't put resources into fixing, but not so trivial it's not worth reporting.

To replicate:

• Create two groups with at least one distinct contact in each and make one the child of another.
• Rebuild your smart group cache. civicrm_group_contact_cache now stores a record for the contacts in the child group so they come up for searches of the parent group.
• Remove the parent/child relationship between the groups.
• NOTE: I'm pretty sure the bug only manifests if the parent group is no longer a parent to ANY other groups.

Expected result:
The child group's contacts are no longer found when searching the parent group.

Actual result:
The child group's contacts show up in searches on the parent group.

It appears that when rebuilding the smart group cache, non-smart non-parent groups are excluded. Which makes sense but excludes the possibility that a group was once a parent group. I think the ideal solution is that when clearing the smart group cache, remove any records where the parent_group_id is neither a parent not smart group. But you could also do this check when removing a parent/child group connection, or even at search time.