remove unnecessary assertion in dns_dispatch_connect()

remove unnecessary assertion in dns_dispatch_connect()

When a thread calls dns_dispatch_connect() on an unconnected TCP socket
it sets `tcpstate` from `DNS_DISPATCHSTATE_NONE` to `_CONNECTING`.
Previously, it then INSISTed that there were no pending connections
before calling isc_nm_tcpdnsconnect().

If a second thread called dns_dispatch_connect() during that window
of time, it could add a pending connection to the list, and trigger
an assertion failure.

This commit removes the INSIST since the condition is actually
harmless.