MdeModulePkg/BMMUiLib: Replace same logic with API in UefiBootManagerLib

Use the API　EfiBootManagerDeleteLoadOptionVariable in UefiBootManagerLib　to
replace the same logic in function Var_DelBootOption/Var_DelDriverOption.
This can make code clean and prevent potential bugs.

https://bugzilla.tianocore.org/show_bug.cgi?id=391

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>