MINOR: mux-quic: remove dedicated function to handle standalone FIN

MINOR: mux-quic: remove dedicated function to handle standalone FIN

Remove QUIC MUX function qcs_http_handle_standalone_fin(). The purpose
of this function was only used when receiving an empty STREAM frame with
FIN bit. Besides, it was called by each application protocol which could
have different approach and render the function purpose unclear.

Invocation of qcs_http_handle_standalone_fin() have been replaced by
explicit code in both H3 and HTTP/0.9 module. In the process, use
htx_set_eom() to reliably put EOM on the HTX message.

This should be backported up to 2.7, along with the previous patch which
introduced htx_set_eom().