QS60Style: Get rid of desktopWidget size bounding rect in CT_ComboBox.

There is a hackish size calculation in QS60Style for comboboxes where the style uses QDesktopWidget to bound a combobox content rect to it.
This is prone to fail, if the desktop widget is not resized yet, when style tries to calculate the combox size.

Note that when fixing this by removing the offending line, make sure that QFileDialogs with really long path names are not broken with QS60Style.