Remove pointless tooltip from Labelbackground

The tooltip simply repeates the text of this component which is short
enough that it's always visible. It is therefore redundant and
unnecessary.


(cherry picked from commit dce22fc7)

discover/qml/LabelBackground.qml

@@ -39,7 +39,4 @@ Control
         horizontalAlignment: Text.AlignHCenter
         color: Kirigami.Theme.highlightedTextColor
     }
-
-    ToolTip.visible: hovered
-    ToolTip.text: theLabel.text
 }