Remove (source,destination,host)_ipv6

There are source_ipv6, etc., fields defined but this is confusing since source_ip can contain ipv6 addresses. This is redundant and will only confuse people who are writing normalization rules.

Search the documentation for the source_ipv6, destination_ipv6, and host_ipv6 field definitions. Remove those redundant entries, then review the surrounding normalization-field documentation to confirm that source_ip, destination_ip, and host_ip remain the documented fields.

Written by the indexing model from the issue text.