dirmngr,build: Remove support for ADNS.

* autogen.rc: Remove '--with-adns' argument.
* configure.ac: Remove check for ADNS.
* dirmngr/dns-stuff.c: Remove all code that uses ADNS.
* dirmngr/server.c (cmd_getinfo): Update status line.
* doc/dirmngr.texi: Do not mention ADNS.

We used ADNS to support queries over Tor. However, our patch to ADNS
that adds Tor support was never accepted upstream. Furthermore, there
are other shortcomings that let us to consider alternatives.

• Signed-off-by: Justus Winter <justus@g10code.com>