Maintainers.txt: Remove Nt32Pkg reference after Nt32Pkg is removed

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Michael D Kinney <michael.d.kinney@intel.com>