Suggested by Maximilien Colange.

* spot/twaalgos/degen.cc: If the output has more SCC than the input,
detect useless SCCs and remove them.
* spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh,
spot/twaalgos/degen.hh: Add support for a degen-remscc option.
* bin/spot-x.cc, NEWS: Document it.
* tests/core/degenscc.test: New file.
* tests/Makefile.am: Add it.
* tests/core/det.test: Lower some expected size (yay!).