[FIX] web: o2m view manager header

Revert commits b3f3c67eea and a40d48378d forward ported from
8.0. In 8.0 these commits removed dead code but it is actually useful
in saas-6.

addons/web/static/src/js/view_form.js

@@ -4363,6 +4363,7 @@ instance.web.form.FieldOne2Many = instance.web.form.AbstractField.extend({
 });
 
 instance.web.form.One2ManyViewManager = instance.web.ViewManager.extend({
+    template: 'One2Many.viewmanager',
     init: function(parent, dataset, views, flags) {
         this._super(parent, dataset, views, _.extend({}, flags, {$sidebar: false}));
         this.registry = instance.web.views.extend({

addons/web/static/src/xml/base.xml

@@ -1589,6 +1589,12 @@
         </t>
     </a>
 </t>
+
+<t t-name="One2Many.viewmanager" t-extend="ViewManager">
+    <t t-jquery=".oe-view-manager-header">
+        this.attr('t-if', 'widget.view_order.length != 1');
+    </t>
+</t>
 <t t-name="One2Many.formview" t-extend="FormView">
     <t t-jquery=".oe_form_buttons" t-operation="inner">
         <button type="button" class="oe_button oe_form_button_create">Add</button>