MdeModulePkg PiSmmCore: Set ForwardLink to NULL in RemoveOldEntry()

"Entry->Link.ForwardLink = NULL;" is present in RemoveMemoryMapEntry()
for DxeCore, that is correct.
"Entry->Link.ForwardLink = NULL;" is absent in RemoveOldEntry()
for PiSmmCore, that is incorrect.

Without this fix, when FromStack in Entry is TRUE,
the "InsertTailList (&mMapStack[mMapDepth].Link, &Entry->Link);" in
following calling to CoreFreeMemoryMapStack() will fail as the entry
at mMapStack[mMapDepth] actually has been removed from the list.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>