Remove custom code for converting from QString to QColor

After commit 9b021a1fbd0b974a452a6e18a8c0b4b97cbf8edb in qtbase we can now
(about a year later ;-) remove the custom code from handling alpha when
converting from string to color and rely on the implementation in qtbase.

Change-Id: Ia8086c99f0f782742d3adf12cb2c3b79dee8d578
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>