GroupchatControl: Fix deprecation warnings

gajim/data/gui/groupchat_control.ui

@@ -166,7 +166,6 @@
                                 <property name="height_request">48</property>
                                 <property name="visible">True</property>
                                 <property name="can_focus">False</property>
-                                <property name="stock">gtk-missing-image</property>
                               </object>
                               <packing>
                                 <property name="expand">False</property>
@@ -362,7 +361,6 @@
                               <object class="GtkImage" id="lock_image">
                                 <property name="visible">True</property>
                                 <property name="can_focus">False</property>
-                                <property name="stock">gtk-dialog-authentication</property>
                                 <property name="icon_size">1</property>
                               </object>
                             </child>
@@ -485,7 +483,7 @@
                             <property name="relief">none</property>
                             <property name="popup">formattings_menu</property>
                             <child>
-                              <object class="GtkImage" id="image10">
+                              <object class="GtkImage">
                                 <property name="visible">True</property>
                                 <property name="can_focus">False</property>
                                 <property name="icon_name">format-text-bold-symbolic</property>