SimicsOpenBoardPkg: Always set the boot priority by default

When running networks of multiple machines, Simics assigns different
disk IDs to each disk created. This change the boot priority and can't
boot from SATA HDD directly.

Clear boot priority in NVRAM can fix this issue.

Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Agyeman Prince <prince.agyeman@intel.com>
Cc: Kubacki Michael A <michael.a.kubacki@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Co-authored-by: Michael Kubacki <michael.a.kubacki@intel.com>
Signed-off-by: David Wei <david.y.wei@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>