twisted: Delete TwistedResolver class

twisted: Delete TwistedResolver class

This class was deprecated and slated for deletion in Tornado 7.0.
However, it has become broken due to the adoption of RFC 8482
(which limits the use of the ANY query type in DNS) and it now fails
for most domain names (with the exception of localhost).
The upstream issue https://github.com/twisted/twisted/issues/10062
has been open for years which is a pretty good sign that no one
is depending on this class and it's safe to remove it ahead of
schedule.

This class was primarily intended to provide thread-free non-blocking
DNS resolution. If that is still required,
`tornado.platform.caresresolver` is the next best option, although it
has its own limitations which differ from TwistedResolver.