two unabbreviate_logic definitions (const and non-const) into a
function that takes a const formula* and return a non-const
formula*.  Since formula* is convertible to const formula*, and
the const version of the function just called the non-onst one, it
makes no sense to keep both.  Also, it confused Swig.
* src/ltlvisit/nenoform.cc, src/ltlvisit/nenoform.hh: Likewise
for negative_normal_form.
* src/ltlvisit/tunabbrev.cc, src/ltlvisit/tunabbrev.hh: Likewise
for unabbreviate_ltl.
* src/ltlvisit/clone.cc, src/ltlvisit/clone.hh: Likewise for clone.
* src/ltlvisit/destroy.cc, src/ltlvisit/destroy.hh: Likewise
for destroy.