As observed in #188, the smaller.hoa automaton is made only of
1-state/1-self-loop SCCs, for which calling remove_fin is a complete
waste of time.  This patch alone (i.e., without the other changes
suggested by #188) improves the run time of

% autofilt -q --is-unambiguous smaller.hoa

from 38s to 0.05s.

* spot/twaalgos/sccinfo.cc: If a single-state SCC has undeterminate SCC
and only one self-loop, then it is necessarily rejecting.
* NEWS: Mention the change.