Don't detach xfr in dns_xfrin_shutdown()
As we are now using dispatch instead of netmgr for XFR TCP connection, the xfrin_recv_done() will be called when cancelling the dispatch with ISC_R_CANCELED. This could lead to double detach from the dns_xfrin_t, one in the xfrin_recv_done() and one in the dns_xfrin_shutdown().
Remove the extra detach from the dns_xfrin_shutdown() and rely on the dispatch read callback to be always called.
Closes #3984 (closed)