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.