Remove unused goto_convertt::has_function_call

Its use (if ever) predates Git history. Removing to reduce the amount of
dead code.

Author: tautschnig

src/goto-programs/goto_convert_class.h

@@ -107,8 +107,6 @@ class goto_convertt:public messaget
 
   void rewrite_boolean(exprt &dest);
 
-  static bool has_function_call(const exprt &expr);
-
   void remove_side_effect(
     side_effect_exprt &expr,
     goto_programt &dest,

src/goto-programs/goto_convert_side_effect.cpp

@@ -22,21 +22,6 @@ Author: Daniel Kroening, kroening@kroening.com
 
 #include <ansi-c/c_expr.h>
 
-bool goto_convertt::has_function_call(const exprt &expr)
-{
-  for(const auto &op : expr.operands())
-  {
-    if(has_function_call(op))
-      return true;
-  }
-
-  if(expr.id()==ID_side_effect &&
-     expr.get(ID_statement)==ID_function_call)
-    return true;
-
-  return false;
-}
-
 void goto_convertt::remove_assignment(
   side_effect_exprt &expr,
   goto_programt &dest,