MINOR: quic: remove unnecessary quic_session_accept()

MINOR: quic: remove unnecessary quic_session_accept()

A specialized listener accept was previously used for QUIC. This is now
unneeded and we can revert to the default one session_accept_fd().

One change of importance is that the call order between
conn_xprt_start() and conn_complete_session() is now reverted to the
default one. This means that MUX instance is now NULL during
qc_xprt_start() and its app-ops layer cannot be set here. This operation
has been delayed to qc_init() to prevent a segfault.

This should be backported up to 2.6.