Delete model on armature node deletion

Previously, the model corresponding to the armature wasn't deleted with
the armature node, causing the armature model to linger around.
Now, they are deleted at the same time as the armature.

See Issue #13924
Merge branch '13924-DeleteArmatureModelWithArmatureNode' into armatures

* 13924-DeleteArmatureModelWithArmatureNode:
  Delete model on armature node deletion