Use default opacity on history page

Otherwise sensors can be hidden beneath one another.

BUG: 434510
FIXED-IN: 5.22

src/pages/history.page

@@ -20,9 +20,6 @@ Title=
 [Face-94107115022400][Sensors]
 highPrioritySensorIds=["memory/physical/used","memory/swap/used"]
 
-[Face-94107115022400][org.kde.ksysguard.linechart][General]
-lineChartFillOpacity=100
-
 [Face-94107150101264][Appearance]
 chartFace=org.kde.ksysguard.linechart
 Title=
@@ -30,9 +27,6 @@ Title=
 [Face-94107150101264][Sensors]
 highPrioritySensorIds=["network/all/download","network/all/upload"]
 
-[Face-94107150101264][org.kde.ksysguard.linechart][General]
-lineChartFillOpacity=100
-
 [page]
 icon=office-chart-line
 margin=2