Use boolbvt for getting counter examples

We use this counter examples in the string solver but the formula given
there don't use arrays so it is enough to use boolbvt.

Author: romainbrenguier

src/solvers/refinement/string_refinement.cpp

@@ -2168,14 +2168,7 @@ static optionalt<exprt> find_counter_example(
   const symbol_exprt &var)
 {
   satcheck_no_simplifiert sat_check;
-  bv_refinementt::infot info;
-  info.ns=&ns;
-  info.prop=&sat_check;
-  info.refine_arithmetic=true;
-  info.refine_arrays=true;
-  info.max_node_refinement=5;
-  info.ui=ui;
-  bv_refinementt solver(info);
+  boolbvt solver(ns, sat_check);
   solver << axiom;
 
   if(solver()==decision_proceduret::resultt::D_SATISFIABLE)