Properly free enemies after death in 3d platformer demo.

Previously, they stuck around as invisible physics objects. This fix is much
cleaner than my previous attempts, as it uses the ability to call a function
from an animation as is done in the 2d platformer demo.

demos/3d/platformer/enemy.gd

@@ -91,4 +91,5 @@ func _ready():
 	# Initalization here
 	pass
 
-
+func _die():
+	queue_free()