Update the ruler annotation layer when deleting a ruler.

Rulers are generated from the ruler annotation layer on tool activation.
The annotation layer is only updated when interacting with the ruler gizmos.
Such as selecting or moving rulers, which is implied when adding a ruler.
Deletion is not part of the gizmo code path, so the annotation layer must be updated explicitly.

Fixes most of T63250 aside from the annotation layer not appearing in the N-panel instantly.
It appears when mousing over the panel, so it seems the panel just needs to be redrawn.