MdeModulePkg SdBlockIoPei: Remove a redundant function

MdeModulePkg SdBlockIoPei: Remove a redundant function

The function SdPeimUnlinkMemBlock that is never called
has been removed.
https://bugzilla.tianocore.org/show_bug.cgi?id=1062

v2: Remove the comment that mentioned 'SdPeimUnlinkMemBlock'
within function SdPeimFreeMemPool().

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: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>