ShellPkg/UefiShellLib: Use BaseLib api CharToUpper

ShellPkg/UefiShellLib: Use BaseLib api CharToUpper

Substitute InternalShellCharToUpper with CharToUpper which is a public
function with the same effect.
Remove the implement of InternalShellCharToUpper.
https://bugzilla.tianocore.org/show_bug.cgi?id=1369

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>