Squash dns_name_fullhash() and dns_name_hash()

The only place where dns_name_hash() was being used is the old hash table in the dns_badcache unit. Squash the dns_name_fullhash() and dns_name_hash() into single dns_name_hash() function that's always case-insensitive as it doesn't make to do case-sensitive hashing of the domain names and we were not using this anywhere.