MdeModulePkg: Refine the code in BootMaintenanceManagerUiLib

Refine the code in function Var_UpdateDriverOption and Var_UpdateBootOption,
use the existed API(EfiBootManagerInitializeLoadOption and
EfiBootManagerAddLoadOptionVariable) supplied by UefiBootManagerLib
to replace the same logic in the two functions. And remove the useless
code.

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