s390/ccwgroup: remove ccwgroup_create_from_string

Remove the old ccwgroup_create_from_string interface since all
drivers have been converted to ccwgroup_create_dev. Also remove
now unused members of ccwgroup_driver.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>