OvmfPkg/PlatformBootManagerLib: Remove dependency on Mps.h

OvmfPkg/PlatformBootManagerLib: Remove dependency on Mps.h

Mps.h is included in BdsPlatform.h but not actually used.
So remove it.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>