CLEANUP: mux-quic: remove useless app_ops is_active callback

CLEANUP: mux-quic: remove useless app_ops is_active callback

Timeout in QUIC MUX has evolved from the simple first implementation. At
the beginning, a connection was considered dead unless bidirectional
streams were opened. This was abstracted through an app callback
is_active().

Now this paradigm has been reversed and a connection is considered alive
by default, unless an error has been reported or a timeout has already
been fired. The callback is_active() is thus not used anymore and can be
safely removed to simplify qcc_is_dead().

This commit should be backported to 2.6.