The distinction makes no sense since Spot 0.5, where we switched from
mutable furmulae to immutable formulae.  The difference between
const_visitor and visitor made no sense either.  They have been merged
into one: visitor.

* iface/dve2/dve2check.cc, iface/gspn/ltlgspn.cc,
src/eltlparse/eltlparse.yy, src/eltlparse/public.hh,
src/evtgbatest/ltl2evtgba.cc, src/kripkeparse/kripkeparse.yy,
src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/automatop.cc, src/ltlast/automatop.hh, src/ltlast/binop.cc,
src/ltlast/binop.hh, src/ltlast/bunop.cc, src/ltlast/bunop.hh,
src/ltlast/constant.cc, src/ltlast/constant.hh, src/ltlast/formula.cc,
src/ltlast/formula.hh, src/ltlast/formula_tree.cc,
src/ltlast/formula_tree.hh, src/ltlast/multop.cc,
src/ltlast/multop.hh, src/ltlast/predecl.hh, src/ltlast/refformula.cc,
src/ltlast/refformula.hh, src/ltlast/unop.cc, src/ltlast/unop.hh,
src/ltlast/visitor.hh, src/ltlenv/declenv.cc, src/ltlenv/declenv.hh,
src/ltlen...