remove_fin: improve behavior on unclean acceptance
Related to #188. This is a third fix that independently makes `'utfilt --is-unambiguous -q smaller.hoa' instantaneous. * spot/twaalgos/remfin.cc: Clean the received automaton if necessary. * bin/autfilt.cc: No need to call cleanup_acceptance_here() before remove_fin() anymore. * tests/core/remfin.test: Add an additional test. * NEWS: Mention the change.