UefiCpuPkg/Include/Register/Msr/P6Msr.h: Remove old MSR.

UefiCpuPkg/Include/Register/Msr/P6Msr.h: Remove old MSR.

Changes includes:
  1. Remove MSR which not existed in 2018-05 version spec: MSR_P6_ROB_CR_BKUPTMPDR6.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>