Remove noop string lookup in symex main loop

Nobody ever sets that option, so the conditional won't be entered.

Author: karkhaz

src/goto-symex/symex_main.cpp

@@ -311,13 +311,7 @@ void goto_symext::symex_step(
   if(!options.get_bool_option("paths"))
     merge_gotos(state);
 
-  // depth exceeded?
-  {
-    unsigned max_depth=options.get_unsigned_int_option("depth");
-    if(max_depth!=0 && state.depth>max_depth)
-      state.guard.add(false_exprt());
-    state.depth++;
-  }
+  state.depth++;
 
   // actually do instruction
   switch(instruction.type)