by default in scc_filter().

Doing so helps the degeneralization algorithm, because it will
have more opportunity to be in an accepting level when it reaches
the accepting SCCs.

* src/tgbaalgos/sccfilter.cc (filter_iter::filter_iter): Take a
remove_all_useless argument.
(filter_iter::process_link): Use the flag to decide whether to
filter acceptance conditions going to accepting SCCs.
(scc_filter): Take a remove_all_useless argument and pass it to
filter_iter.
* src/tgbaalgos/sccfilter.hh (filter_iter): Add the new argument
and document the function.
* src/tgbatest/tgbatests/ltl2tgba.cc (main): Add option use -R3
for remove_all_useless=false and add -R3f for
remove_all_useless=true.
* src/tgbatest/ltl2tgba.test: Show one case where -R3f makes
the degeneralization worse than -R3.