Remove unused variables

Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV103
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV104
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV105
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV107
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV108
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV334
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV481
Task-number: QTBUG-146050
Pick-to: 6.12
Change-Id: I308315a5ed8e61235d39d58336912b80bd64b1b0
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Kwanghyo Park <kwanghyo.park@qt.io>