[FIX] web: m2o field already extends from ServicesMixin

FieldMany2One was declared as an extension of AbstractRelationalField
AND mixins.ServicesMixin... which was wrong since the mixin is already
added to the Widget class. This caused the extension of the mixin
methods to be lost.

addons/web/static/src/js/fields/relational_fields.js

@@ -25,7 +25,6 @@ var Dialog = require('web.Dialog');
 var field_utils = require('web.field_utils');
 var KanbanRenderer = require('web.KanbanRenderer');
 var ListRenderer = require('web.ListRenderer');
-var mixins = require('web.mixins');
 var Pager = require('web.Pager');
 var pyeval = require('web.pyeval');
 
@@ -105,7 +104,7 @@ var M2ODialog = Dialog.extend({
     },
 });
 
-var FieldMany2One = AbstractRelationalField.extend(mixins.ServicesMixin, {
+var FieldMany2One = AbstractRelationalField.extend({
     template: 'FieldMany2One',
     custom_events: {
         'quick_create': '_onQuickCreate',