MinPlatformPkg: Remove the global variable mForceX2ApicId

This global variable mForceX2ApicId is not assigned
to any value in code and will be initialized to 0 when
the driver is loaded. So remove the global variable
and related code that will not be executed.

Signed-off-by: Dun Tan <dun.tan@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Cc: Jason Lou <yun.lou@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Eric Dong <eric.dong@intel.com>