call-window: Remove superfluous GtkViewport

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

src/ui/call-window.ui

@@ -14,8 +14,6 @@
                 <property name="child">
                   <object class="GtkScrolledWindow">
                     <property name="hscrollbar_policy">never</property>
-                    <property name="child">
-                      <object class="GtkViewport">
                     <property name="child">
                       <object class="GtkFlowBox" id="call_selector">
                         <property name="orientation">vertical</property>
@@ -29,8 +27,6 @@
                   </object>
                 </property>
               </object>
-                </property>
-              </object>
             </child>
             <child>
               <object class="GtkStackPage">