Xserver/os/xprintf.c: Drop NoMachine's own implementation of Xvasprintf(). Use stock implementation instead.

 * Reason from NoMachine's NX CHANGELOG for having its own implementation
   of Xvasprintf() (from around nxagent 3.3.0):

    - Fixed TR06G02225. The implementation of Xvprintf() has been reviewed
      to work on more platforms. Previous implementation caused a failure
      in the build of keyboard map on some platform like Solaris 8 and 9.
    - Fixed TR03G02198. Reimplemented Xvprintf() in Xserver/os to handle
      the case in which vsnprintf returned -1.

 * Reason for removing it again:

    - Fixes segfaults in SetDefaultFontPath when launching nxagent.
    - All locations in Xserver/** using the code check for return value
      of -1.
    - Solaris 8 and 9 are beyond of our support scope.
    - Keep more in sync with X.org.
    - Good moment to drop more GPL-2 code from Xserver's code base.