CDK-800: Remove csv-import schema validation check.

This removes the step in csv-import that validates the apparent schema
of the file with the target dataset's schema. The only way to validate
CSV is to validate each row, which is already done during the import
when constructing valid Avro records.
Actually, I now remember that I left this in because removing it will cause scripts that use it to fail. Instead, I've added a warning that states the option will be removed in 0.19.0.