Unset possible ID column to prevent unwanted behaviour

If the ID column isn't unset, then `wp_insert_user` will return this ID while not having created a new user. Therefor passing the `is_wp_error` check and showing the success message without actually having created a new user.
Merge pull request #961 from wp-cli/import-users-with-id-column

wp user import-csv users.csv with ID column