Maintainers.txt: Remove DuetPkg

Maintainers.txt: Remove DuetPkg

Since DuetPkg is due to be removed, Maintainers.txt
should also be updated.
https://bugzilla.tianocore.org/show_bug.cgi?id=1322

Cc: Hao Wu <hao.a.wu@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>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>