MINOR: listener: remove the now useless LI_F_QUIC_LISTENER flag

MINOR: listener: remove the now useless LI_F_QUIC_LISTENER flag

This flag is only used to tag a QUIC listener, which we now know by
its bind_conf's xprt as well. It's only used to decide whether or not
to perform an extra initialization step on the listener. Let's drop it
as well as the flags field.

With the various fields and options moved, the listener struct reduced
by 48 bytes total.