[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/models/contract_line.py

@@ -44,9 +44,9 @@ class AccountAnalyticInvoiceLine(models.Model):
         return quantity
 
     @api.multi
-    def _prepare_invoice_line(self, invoice_id=False):
+    def _prepare_invoice_line(self, invoice_id=False, invoice_values=False):
         vals = super(AccountAnalyticInvoiceLine, self)._prepare_invoice_line(
-            invoice_id=invoice_id
+            invoice_id=invoice_id, invoice_values=invoice_values,
         )
         if (
             'quantity' in vals