Don't re-declare the color property

QXYSeries already declares color as a property, with a virtual setter
and getter that QScatterSeries than overrides.

Discovered while investigating qt_Add-ons_qtcharts_dev_linux:SV536. We
probably cannot remove the redeclared colorChanged signal as this would
remove the symbol from the ABI.

Task-number: QTBUG-146050
Pick-to: 6.12
Change-Id: I31b6cb83d65f5447e1b9aed5d09ffaecee0119ea
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>