Storageclass does not vanish with deleted arangostorage deployment

[neunhoef]

When one deploys an arangostorage resource with the storage operator, then the corresponding storage class is automatically generated. However, if one deletes the arangostorage resource again, the storage class remains.

Note that test4a.fish and test4b.fish currently clean up "manually". Once this issue is resolved, this should be changed as well.