* src/ltlvisit/simplify.hh, src/ltlvisit/simplify.cc
(ltl_simplify::negative_normal_form): Remove the third
parameter and always rewrite XOR, =>, and <=>.  This avoid
problems with the cache, that could have been populated with
a different value for this third parameter.
* src/ltltest/reduc.cc, src/tgbaalgos/ltl2tgba_fm.cc: Adjust
calls to negative_normal_form().
* src/ltltest/nenoform.test: Adjust tests.