[FIX] point_of_sale: show the invoice button for paid orders

Introduced at e434690fe3
The button should be displayed when the order is paid to generate the invoice
Once the invoice is generated, the state is invoiced and a statbutton lead to
the generated invoice.

Remove the readonly attrs as it has no effect on a button

addons/point_of_sale/views/pos_order_view.xml

@@ -7,7 +7,8 @@
             <form string="Point of Sale Orders" create="0">
                 <header>
                     <button name="%(action_pos_payment)d" string="Payment" class="oe_highlight" type="action" states="draft" context="{'pos_session_id' : session_id}"/>
-                    <button name="action_pos_order_invoice" string="Invoice" type="object" attrs="{'invisible': ['|', ('invoice_group', '=', False), ('state','!=','invoiced')], 'readonly': [('partner_id','=',False)]}"/>
+                    <button name="action_pos_order_invoice" string="Invoice" type="object"
+                            attrs="{'invisible': ['|', ('invoice_group', '=', False), ('state','!=','paid')]}"/>
                     <button name="refund" string="Return Products" type="object"
                         attrs="{'invisible':[('state','=','draft')]}"/>
                     <field name="state" widget="statusbar" statusbar_visible="draft,paid,done" />