Fix group on member unmanaged

Previously, the unmanaged entity would stay a member of the group.
On rebind, this would fail (because the entity was a dangling reference). The actual error was caused by the attribute with the members being an ImmutableSet, so putting null in it failed.

Now the unmanaged entity is immediately removed from all of its groups.
If this is the case maybe method should move to `EntityInternal` ?  But not in this PR.