PurleyOpenBoardPkg/BoardMtOlympus: Use shell source directly

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1675



In long term we will remove ShellBinPkg, so now update
platform to use ShellPkg directly.

Cc: Shifei A Lu <shifei.a.lu@intel.com>
Cc: Xiaohu Zhou <bowen.zhou@intel.com>
Cc: Isaac W Oram <isaac.w.oram@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Xiaohu Zhou <bowen.zhou@intel.com>