MdeModulePkg/BootMaintenanceManagerUiLib: remove unused but set variable
As-is, the code triggers [-Werror=unused-but-set-variable] with GCC.
Fixes: a85be3ae48a8aaa40b755cd0ff7270c67cfed585
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>