BUG/MEDIUM: quic: always remove the connection from the accept list on close

BUG/MEDIUM: quic: always remove the connection from the accept list on close

Fred managed to reproduce a crash showing a corrupted accept_list when
firing thousands of concurrent picoquicdemo clients to a same instance.
It may happen if the connection was placed into the accept_list and
immediately closed before being processed (e.g. on error or t/o ?).

In any case the quic_conn_release() function should always detach a
connection to be deleted from any list, like it does for other lists,
so let's add an MT_LIST_DELETE() here.

This should be backported to 2.6.