Deprecate org.apache.cassandra.hadoop code

The code under org.apache.cassandra.hadoop is becoming a legacy and probably should be deleted.

The code does not have any example / test how to use it, it is not documented and it depends on Hadoop of version 1 which is hardly used nowadays.

This ticket should track its deletion.

Feel free to object and we might revisit this decision. If we remove it in 4.1, it will still be present in 4.0.x where we can still patch.