Remove now redundant import

Author: Joel Allred

jbmc/unit/solvers/strings/string_refinement/dependency_graph.cpp

@@ -10,7 +10,6 @@ Author: Diffblue Ltd.
 #include <testing-utils/use_catch.h>
 
 #include <java_bytecode/java_types.h>
-#include <solvers/strings/string_refinement_util.h>
 #include <solvers/strings/string_dependencies.h>
 #include <util/arith_tools.h>
 #include <util/std_expr.h>