Based on a report by Emmanuel Filiot, who was surprized that dualizing
Büchi did not always produce co-Büchi.

* spot/twaalgos/dualize.cc: Remove the call to
cleanup_acceptance_here.
* spot/twaalgos/dualize.hh: Improve documentation.
* NEWS: Mention the possible backward incompatible change.
* tests/core/dualize.test, tests/python/dualize.py,
tests/python/pdegen.py: Adjust test cases.
* spot/twaalgos/complement.cc (complement): Call
cleanup_acceptance_here when dualize() returns a smaller automaton.
* THANKS: Add Emmanuel.