[FIX] web: BS4, restore slider checkboxes

Take advantage of the new BS4 checkbox structure to simplify the
widget too.

addons/web/static/tests/fields/basic_fields_tests.js

@@ -359,7 +359,7 @@ QUnit.module('basic_fields', {
     QUnit.module('FieldBooleanToggle');
 
     QUnit.test('use boolean toggle widget in form view', function (assert) {
-        assert.expect(2);
+        assert.expect(1);
 
         var form = createView({
             View: FormView,
@@ -370,7 +370,6 @@ QUnit.module('basic_fields', {
         });
 
         assert.strictEqual(form.$(".custom-checkbox.o_boolean_toggle").length, 1, "Boolean toggle widget applied to boolean field");
-        assert.strictEqual(form.$(".custom-checkbox.o_boolean_toggle").find(".slider").length, 1, "Boolean toggle contains slider to toggle");
         form.destroy();
     });