MdeModulePkg XhciDxe: Remove redundant functions

The functions that are never called have been removed.
They are XhcReadDoorBellReg and XhcWriteOpReg16.
https://bugzilla.tianocore.org/show_bug.cgi?id=1062

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: shenglei <shenglei.zhang@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>