Operator never removes member-of label from SMCP namespace

When you create an SMCP, the operator adds the `maistra.io/ignore-namespace`and `maistra.io/member-of` labels to the SMCP's namespace. When you delete the SMCP, neither of these labels is removed.

This can cause issues later, because you can't add the namespace as a member to any other service mesh. If you do, the controller generates an event saying that the namespace is already part of a different mesh. The namespace itself is actually in no SMMR, leaving users confused. The left-over `member-of` label is causing the issue.