Cleanup zloop working directory after each pass

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: John Kennedy <jwk404@gmail.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Issue openzfs#6595 
Closes openzfs#6663

scripts/zloop.sh

@@ -181,7 +181,6 @@ while getopts ":ht:m:s:c:f:l" opt; do
 		s ) [[ $OPTARG ]] && size=$OPTARG ;;
 		c ) [[ $OPTARG ]] && coredir=$OPTARG ;;
 		f ) [[ $OPTARG ]] && basedir=$(readlink -f "$OPTARG") ;;
-		l ) symlink=1 ;;
 		h ) usage
 		    exit 2
 		    ;;