Remove dnsserver and external DNS helper API

Remove dnsserver and external DNS helper API

The external DNS helper API places limits on Squid DNS lookups per second
low enough to noticably affect the HTTP requests per second which can be
served.

Request for comments about proposed removal 2 years ago produced feedback
stating that the helper was needed for local name resolution. This is
now available since 3.4 mDNS extensions.

A more recent request for reasons for keeping the helper API have
produced only two responses over the period of several months. Both
indicating that the API is no longer necessary for the business cases of
a year or so ago.

As such and because the helper fails to operate sufficiently on several
major operating systems and the API is difficult to maintain it is being
removed as of Squid-3.5.