MdeModulePkg/BMMUiLib: Remove old useless data before new save action

MdeModulePkg/BMMUiLib: Remove old useless data before new save action

There exits the case that when saving changes in form A,
the old saved data in form B are not cleaned, will be saved
again with the new save. Thus incorrect UI behavior will be
shown. This patch is to remove some useless data.

https://bugzilla.tianocore.org/show_bug.cgi?id=385

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>