GCTooltip: Fix deprecation warning

gajim/data/gui/tooltip_gc_contact.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.22.1 -->
+<!-- Generated with glade 3.36.0 -->
 <interface>
-  <requires lib="gtk+" version="3.20"/>
+  <requires lib="gtk+" version="3.22"/>
   <object class="GtkGrid" id="tooltip_grid">
     <property name="visible">True</property>
     <property name="can_focus">False</property>
@@ -10,7 +10,6 @@
       <object class="GtkImage" id="avatar">
         <property name="visible">True</property>
         <property name="can_focus">False</property>
-        <property name="stock">gtk-missing-image</property>
       </object>
       <packing>
         <property name="left_attach">0</property>