When deleting `BMC`s the `ServerMaintenance` is not being deleted

[SchwarzM]

Describe the bug
When I delete BMC's to rediscover the associated server maintenance is orphaned and kept around

To Reproduce
Create a BMC and a server maintenance for it. Delete the bmc.

Expected behavior
The server maintenance should have an owner reference on the bmc and be deleted along with the bmc

[stefanhipfel]

will be fixed with #272