treewide: Remove unused placeholder elements

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/714>

src/ui/account-overview.ui

@@ -62,7 +62,6 @@
                         <property name="margin-start">12</property>
                         <property name="margin-end">12</property>
                         <signal name="row-activated" handler="on_account_row_activated" swapped="no"/>
-                        <child type="placeholder"/>
                         <style>
                           <class name="boxed-list"/>
                         </style>

src/ui/call-selector-item.ui

@@ -29,14 +29,8 @@
                 </child>
               </object>
             </child>
-            <child>
-              <placeholder/>
-            </child>
           </object>
         </property>
-        <child type="label_item">
-          <placeholder/>
-        </child>
       </object>
     </child>
   </template>