Do not use state Function field of line to check against modification

The Function field is an helper to write PYSON statement so there is no need to
use it for checks.

Closes #14002

modules/purchase/purchase.py

@@ -2125,7 +2125,7 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
                             purchase=line.purchase.rec_name))
         elif mode == 'delete':
             for line in lines:
-                if line.purchase_state not in {'cancelled', 'draft'}:
+                if line.purchase.state not in {'cancelled', 'draft'}:
                     raise AccessError(gettext(
                             'purchase.msg_purchase_line_delete_cancel_draft',
                             line=line.rec_name,

modules/sale/sale.py

@@ -2258,7 +2258,7 @@ class SaleLine(TaxableMixin, sequence_ordered(), ModelSQL, ModelView):
                             sale=line.sale.rec_name))
         elif mode == 'delete':
             for line in lines:
-                if line.sale_state not in {'cancelled', 'draft'}:
+                if line.sale.state not in {'cancelled', 'draft'}:
                     raise AccessError(gettext(
                             'sale.msg_sale_line_delete_cancel_draft',
                             line=line.rec_name,