clean out --opensmt command line option

Support was dropped in April 2018.

Author: Daniel Kroening

src/cbmc/cbmc_parse_options.h

@@ -46,7 +46,7 @@ class optionst;
   OPT_GOTO_CHECK \
   "(no-assertions)(no-assumptions)" \
   "(xml-ui)(xml-interface)(json-ui)" \
-  "(smt1)(smt2)(fpa)(cvc3)(cvc4)(boolector)(yices)(z3)(opensmt)(mathsat)" \
+  "(smt1)(smt2)(fpa)(cvc3)(cvc4)(boolector)(yices)(z3)(mathsat)" \
   "(cprover-smt2)" \
   "(no-sat-preprocessor)" \
   "(beautify)" \