[FIX] mass_mailing: do not define xml attribute twice

Oversight of previous forward-port.

addons/mass_mailing/views/mass_mailing_views.xml

@@ -649,7 +649,7 @@
                                         <field name="campaign_id"
                                             string="UTM Campaign"
                                             groups="base.group_no_one"/>
-                                        <field name="source_id" readonly="1"
+                                        <field name="source_id"
                                             readonly="1"
                                             string="Source"
                                             required="False"