Make sure inherited memberships are not themselves inherited

Make sure inherited memberships are not themselves inherited

In trying to make sense of the code / fix / test for https://github.com/civicrm/civicrm-core/pull/15062
I discovered the tests wouldn't pass due to a weird edge case where an individual inherited a membership
and that membership was inherited in turn via a relationship the individual had (with the same organization)

I don't believe that inheriting inherited memberships is an intentional feature & it DOEs cause issues
- this removes that possibility