Resolve #3022: DoH: dig eventually aborts on ALPN negotiation failure when issuing a DoH query (because of dangling handles)

This commit removes unneeded isc__nmsocket_prep_destroy() call on ALPN negotiation failure, which was eventually causing the TLS handle to leak.

This call is not needed, as not attaching to the transport (TLS) handle should be enough. At this point it seems like a kludge from earlier days of the TLS code.

Closes #3022 (closed)