Enable LTL simplifications by default for ltl2tgba & ltl2tgta, and make
sure the ltl_simplifier_options are all false initially.  Before this
patch --low/-r1 had the same effect as --medium/-r2 with respect to LTL
simplification.

* src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc (simplification_level): Set
to 3 by default.
* src/bin/common_r.cc: Disable all ltl_simplifier options initially.