[QStaticTextItem] Drop unused members

The QStaticTextItem's `chars` and `numChars` members are never used,
even when set. In fact, there is nothing useful we can do with the input
string in the absence of clusters mapping.

Change-Id: I81fcbce3e575f4d0dcfc2515286d512b858f592b
Reviewed-by: Lars Knoll <lars.knoll@digia.com>