[FIX] calendar: no add Contact when no useContacts

On a form view the attribute `use_contacts` will:

- when set to True: enable us to add contacts which will be available on
  any time in the calendar,

- when unset: the list of contact is only the contact currently beeing
  seen directly in the calendar.

The feature for adding contact is thus not necessary when
`use_contacts` is unset. But it still appeared without having any effect
(which is unexpected).

This commit remove the contact adding feature when it has no necessity.

opw-715932

addons/calendar/static/src/js/base_calendar.js

@@ -116,6 +116,9 @@ widgets.SidebarFilter.include({
     },
     initialize_m2o: function() {
         var self = this;
+        if (!this.view.useContacts) {
+            return;
+        }
         this.dfm = new form_common.DefaultFieldManager(self);
         this.dfm.extend_field_desc({
             partner_id: {