Allow to delete invoice lines

As now they will be recreated by purchase module, there is no problem with
deleting them, so we can allow it.

issue6131
review30921002

__init__.py

@@ -3,14 +3,12 @@
 
 from trytond.pool import Pool
 from .purchase import *
-from .invoice import *
 
 
 def register():
     Pool.register(
         Purchase,
         PurchaseIgnoredInvoiceLine,
-        InvoiceLine,
         module='purchase_invoice_line_standalone', type_='model')
     Pool.register(
         HandleInvoiceException,

invoice.py

@@ -1,26 +0,0 @@
-# This file is part of Tryton.  The COPYRIGHT file at the top level of
-# this repository contains the full copyright notices and license terms.
-from trytond.pool import Pool, PoolMeta
-
-__all__ = ['InvoiceLine']
-
-
-class InvoiceLine:
-    __metaclass__ = PoolMeta
-    __name__ = 'account.invoice.line'
-
-    @classmethod
-    def __setup__(cls):
-        super(InvoiceLine, cls).__setup__()
-        cls._error_messages.update({
-                'delete_purchase_invoice_line': ('You can not delete '
-                    'invoice lines that comes from a purchase.'),
-                })
-
-    @classmethod
-    def delete(cls, lines):
-        PurchaseLine = Pool().get('purchase.line')
-        if any(l for l in lines
-                if isinstance(l.origin, PurchaseLine) and l.type == 'line'):
-            cls.raise_user_error('delete_purchase_invoice_line')
-        super(InvoiceLine, cls).delete(lines)