remove unusuable goto_programt::make_return variant

The instruction generated by this variant violates an invariant, and thus,
this method is not usable.

Author: kroening

src/goto-programs/goto_program.h

@@ -845,12 +845,6 @@ class goto_programt
     }
   }
 
-  static instructiont
-  make_return(const source_locationt &l = source_locationt::nil())
-  {
-    return instructiont(code_returnt(), l, SET_RETURN_VALUE, nil_exprt(), {});
-  }
-
   static instructiont make_return(
     code_returnt c,
     const source_locationt &l = source_locationt::nil())

unit/goto-instrument/cover_instrument.cpp

@@ -17,7 +17,7 @@ TEST_CASE("cover_intrument_end_of_function", "[core]")
     goto_programt::make_function_call(code_function_callt({}, {})));
   goto_program.add(
     goto_programt::make_function_call(code_function_callt({}, {})));
-  goto_program.add(goto_programt::make_return());
+  goto_program.add(goto_programt::make_end_function());
   // Act
   cover_instrument_end_of_function(
     "foo", goto_program, goto_programt::make_assertion);
@@ -37,7 +37,7 @@ TEST_CASE("cover_instrument_end_of_function with custom expression", "[core]")
     goto_programt::make_function_call(code_function_callt({}, {})));
   goto_program.add(
     goto_programt::make_function_call(code_function_callt({}, {})));
-  goto_program.add(goto_programt::make_return());
+  goto_program.add(goto_programt::make_end_function());
   const cover_instrumenter_baset::assertion_factoryt assertion_factory =
     [](const exprt &, const source_locationt &location) {
       return goto_programt::make_assertion(true_exprt{}, location);

unit/goto-programs/goto_program_validate.cpp

@@ -197,7 +197,8 @@ SCENARIO("Validation of a goto program", "[core][goto-programs][validate]")
       auto &function_map = goto_model.goto_functions.function_map;
       auto it = function_map.find("g");
       auto &instructions = it->second.body.instructions;
-      instructions.insert(instructions.begin(), goto_programt::make_return());
+      instructions.insert(
+        instructions.begin(), goto_programt::make_return(code_returnt()));
 
       goto_model_validation_optionst validation_options{
         goto_model_validation_optionst ::set_optionst::all_false};