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.
Merge pull request #270 from rdblue/CDK-800-fix-csv-schema-validation

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