Remove the fixed font size on License page on Linux

Use the system settings instead like on Windows and Mac.
Stops text from becoming illegible on high dpi displays.

Task-number: QTIFW-437
Change-Id: Ieca4eba2a24d97ffb5b211623ce8883c011d33da
Reviewed-by: Kai Koehne <kai.koehne@digia.com>