ShellPkg: Don't strip BOM when piping UCS2 data to a UEFI app

ShellPkg: Don't strip BOM when piping UCS2 data to a UEFI app

If the BOM is stripped from StdIn, then an app that duplicates StdIn
will not be able to duplicate, say, a UCS2 file that was piped into
it (the output file it creates would not start with a BOM).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jim Dailey <jim_dailey@dell.com>
Reviewed-by: Jaben Carsey <Jaben.Carsey@intel.com>