MdeModulePkg/BootMaintenanceManagerUiLib: Remove type casting in ChooseFile
The type casting is not necessary and now remove it.
Cc: Liming Gao <liming.gao@intel.com> Cc: Eric Dong <eric.dong@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> Reviewed-by: Gary Lin <glin@suse.com>