Maintainers.txt: Remove Network maintainers for MdeModulePkg/Universal/Network

MdeModulePkg/Universal/Network has been moved to NetworkPkg. So,
MdeModulePkg/Universal/Network can be removed in Maintainers.txt.

Signed-off-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>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>