ShellPkg: refactor elimination of non-replaced environment variables

This changes how non-replaced environment variables are found and eliminated from the command line.  This new method makes sure that the found environment variables are not using escaped characters and that they do not stretch over quoted strings

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@15242 6f19259b-4bc3-4df7-8a09-765794883524