[FIX] supplier invoice: avoid discount doubloon

Standard discount is only visible when you active "Allow setting a discount on the sales order lines" in Sales Configuration.
If you active this option + install this addon : the discount is visible two times on invoice lines (only for supplier invoices).
This commit allows to display only one field in all cases.
Merge pull request #33 from rdeheele/7.0-fix_doubloon_discount_field

[FIX] supplier invoice: avoid discount doubloon