[FIX] payment_paypal: do not compute field `paypal_url`

Since 85bcd6fd, the field `paypal_account`
has been removed. The field `paypal_url` cannot be computed correctly
anymore. Convert it to a dummy field.

addons/payment_paypal/models/account_invoice.py

 # -*- coding: utf-8 -*-
 
-from openerp import models, fields, api
-
-import werkzeug.urls
+from openerp import models, fields
 
 class AccountInvoice(models.Model):
     _inherit = 'account.invoice'
 
-    paypal_url = fields.Char('Paypal Url', compute='_compute_paypal_url')
-
-    @api.depends('type', 'number', 'company_id', 'currency_id')
-    def _compute_paypal_url(self):
-        for inv in self:
-            if inv.type == 'out_invoice' and inv.company_id.paypal_account:
-                params = {
-                    "cmd": "_xclick",
-                    "business": inv.company_id.paypal_account,
-                    "item_name": "%s Invoice %s" % (inv.company_id.name, inv.number or ''),
-                    "invoice": inv.number,
-                    "amount": inv.residual,
-                    "currency_code": inv.currency_id.name,
-                    "button_subtype": "services",
-                    "no_note": "1",
-                    "bn": "Odoo_Invoice_PayNow_" + inv.currency_id.name,
-                }
-                inv.paypal_url = "https://www.paypal.com/cgi-bin/webscr?" + werkzeug.url_encode(params)
+    paypal_url = fields.Char('Paypal Url', store=False)     # dummy field