unit_morph.lua & unit_carrier_drones.lua: cleanup of morph handling.

Instead of unit_morph.lua call unit_carrier_drones.lua to inform about morph, unit_carrier_drones.lua will wait for UnitDestroyed() event (like Jump gadget do) to check for morph.

The reason for this change is because previous method require back-and-forth communication, while in later method unit_carrier_drones.lua only check morph info from unit_morph.lua once. Later method felt more self-contained.