[FIX] hr_holidays: fix hr_presence_status widget

Before this commit, when the employee is off (because he is on leave)
then the presence status icon displayed in org chart, kanban and form view
should be `fa-plan`. It is not the case because `fa-circle` is also added
by mistake.

This commit changes the patch made in `hr_holidays` module to correctly
replace the icon by the `fa-plane` when the employee is on leave.

task-3556262

Part-of: odoo/odoo#138803

addons/hr_holidays/static/src/components/hr_presence_status/hr_presence_status.js

@@ -5,12 +5,11 @@ import { patch } from "@web/core/utils/patch";
 import { HrPresenceStatus } from "@hr/components/hr_presence_status/hr_presence_status";
 
 patch(HrPresenceStatus.prototype, {
-    get classNames() {
-        const classNames = super.classNames;
+    get icon() {
         if (this.value.startsWith("presence_holiday")) {
-            return `${classNames} fa-plane`;
+            return "fa-plane";
         }
-        return classNames;
+        return super.icon;
     },
 
     get color() {