Fixes#333.
* spot/twaalgos/remfin.cc, spot/twaalgos/remfin.hh,
spot/twaalgos/totgba.cc: Adjust. The assert() added
to remove_fin() triggered a lot of failure in the test
suite before the different functions were fixed.
* tests/core/remfin.test, tests/python/tra2tba.py:
Adjust expected result.
* NEWS: Mention the bug.