[IMP] contract: remove redundant method

I have detected a method that was created as redundant and with the same
technique used when preparing the line values, so better to have
everything together in the same method instead of having it spread.

contract_variable_quantity/__manifest__.py

@@ -5,7 +5,7 @@
 
 {
     'name': 'Variable quantity in contract recurrent invoicing',
-    'version': '12.0.2.1.1',
+    'version': '12.0.2.1.2',
     'category': 'Contract Management',
     'license': 'AGPL-3',
     'author': "Tecnativa,"