messagelistview: hide "Scroll to end" after click

When scrolling up and clicking on "Scroll to end", sometimes, the
button didn't update correctly. Using contentY = 0 seems to be correct

Change-Id: I04576181d0b61006d4389be6ace3b09c1ac0a2f9

src/app/mainview/components/MessageListView.qml

@@ -286,8 +286,7 @@ JamiListView {
         anchors.horizontalCenter: root.horizontalCenter
 
         activeStateTrigger: Math.abs(root.contentY) > root.height * 2
-        onClicked: root.ScrollBar.vertical.position =
-                   1.0 - root.ScrollBar.vertical.size
+        onClicked: root.contentY = 0
     }
 
     header: Control {