MINOR: mux-quic: remove qcc_shutdown() from qcc_release()

MINOR: mux-quic: remove qcc_shutdown() from qcc_release()

Render qcc_release() more generic by removing qcc_shutdown(). This
prevents systematic graceful shutdown/CONNECTION_CLOSE emission if only
QCC resource deallocation is necessary.

For now, qcc_shutdown() is used before every qcc_release() invocation.
The only exception is on qmux_destroy stream layer callback.

This commit will be useful to reuse qcc_release() in other contexts to
simply deallocate a QCC instance.