BaseTools/CommonLib: drop definition of MAX_UINTN

BaseTools/CommonLib: drop definition of MAX_UINTN

The maximum value that can be represented by the native word size
of the *target* should be irrelevant when compiling tools that
run on the build *host*. So drop the definition of MAX_UINTN, now
that we no longer use it.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>