MINOR: mux-quic: refactor transport parameters init

MINOR: mux-quic: refactor transport parameters init

Since QUIC accept handling has been improved, the MUX is initialized
after the handshake completion. Thus its safe to access transport
parameters in qc_init via the quic_conn.

Remove quic_mux_transport_params_update which was called by the
transport for the MUX. This improves the architecture by removing a
direct call from the transport to the MUX.

The deleted function body is not transfered to qc_init because this part
will change heavily in the near future when implementing the
flow-control.