This was only used in ELTL stuff, which I just removed because it was
unused.

* src/ltlast/automatop.cc, src/ltlast/automatop.hh,
src/ltlast/formula_tree.cc, src/ltlast/formula_tree.hh,
src/ltlast/nfa.cc, src/ltlast/nfa.hh: Delete.
* src/ltlast/Makefile.am: Adjust.
* src/ltlast/allnodes.hh, src/ltlast/formula.hh, src/ltlast/predecl.hh,
src/ltlast/visitor.hh, src/ltltest/equals.cc, src/ltltest/ltlrel.cc,
src/ltltest/reduc.cc, src/ltlvisit/clone.cc, src/ltlvisit/clone.hh,
src/ltlvisit/dotty.cc, src/ltlvisit/lbt.cc, src/ltlvisit/mark.cc,
src/ltlvisit/postfix.cc, src/ltlvisit/postfix.hh,
src/ltlvisit/relabel.cc, src/ltlvisit/simplify.cc, src/ltlvisit/snf.cc,
src/ltlvisit/tostring.cc, src/tgba/formula2bdd.cc,
src/tgbaalgos/ltl2taa.cc, src/tgbaalgos/ltl2tgba_fm.cc,
src/tgbaalgos/ltl2tgba_lacim.cc, src/tgbatest/ltl2tgba.cc,
iface/dve2/dve2check.cc: Remove all references to automatop.