ShellPkg: fix DmpStore for deleting

This change allows DmpStore command to delete all variables whereas before it would stop after a single deletion due to looping errors.  It uses a recursive function and deletes the last item returned from GetNextVariableName() first and then goes "backwards" to the first items.  

This can't delete authenticated variables.

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: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15014 6f19259b-4bc3-4df7-8a09-765794883524