Small cleanup for CSysInfo

Right now CSysInfo has many function for checking Windows version.
This cleanup removes specialized function 'IsVistaOrHigher' and 'IsWindows8OrHigher' and replace it usage with general function 'IsWindowsVersionAtLeast'

Hopefully in future CSysInfo will not be spoiled with additional functions like 'IsWindows7OrLater', 'IsWindowsBlueOrLater'...