SimicsOpenBoardPkg: Remove un-used PlatformBootManagerLib

Signed-off-by: Ray Ni <ray.ni@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Zhiguang Liu <zhiguang@liu@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>