Don't generate empty doc sections and reduce code duplication

doc/classes/BoxMesh.xml

@@ -10,8 +10,6 @@
 	</description>
 	<tutorials>
 	</tutorials>
-	<methods>
-	</methods>
 	<members>
 		<member name="size" type="Vector3" setter="set_size" getter="get_size" default="Vector3(2, 2, 2)">
 			The box's width, height and depth.
@@ -26,6 +24,4 @@
 			Number of extra edge loops inserted along the X axis.
 		</member>
 	</members>
-	<constants>
-	</constants>
 </class>