Maintainers.txt: Update maintainers for Simics packages

David Y Wei is no longer available as a maintainer. David is removed
as a maintainer for the following packages:
 * Platform/Intel/SimicsOpenBoardPkg
 * Silicon/Intel/SimicsIch10Pkg
 * Silicon/Intel/SimicsX58SktPkg

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Agyeman Prince <prince.agyeman@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Prince Agyeman <prince.agyeman@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>