UnixPkg: Remove UnixPkg and update Maintainers.txt

UnixPkg: Remove UnixPkg and update Maintainers.txt

UnixPkg is deprecated.
The same functionality is supported using the EmulatorPkg.
Please see EmulatorPkg/README for more information about
EmulatorPkg.
https://bugzilla.tianocore.org/show_bug.cgi?id=1291

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: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>