remove AudioEngine::destroy() calls from various test/util code

ARDOUR::cleanup() does this, and the engine must exist during port deregistration.

headless/load_session.cc

@@ -275,6 +275,6 @@ main (int argc, char* argv[])
 	delete s;
 	AudioEngine::instance ()->stop ();
 
-	AudioEngine::destroy ();
+	ARDOUR::cleanup ();
 	return 0;
 }

libs/ardour/test/audio_engine_test.cc

@@ -36,8 +36,6 @@ AudioEngineTest::test_backends ()
 		i != backends.end(); ++i) {
 		print_audio_backend_info(*i);
 	}
-
-	AudioEngine::destroy ();
 }
 
 void
@@ -56,6 +54,4 @@ AudioEngineTest::test_start ()
 	Glib::usleep(2000);
 
 	CPPUNIT_ASSERT (engine->stop () == 0);
-
-	AudioEngine::destroy ();
 }

libs/ardour/test/load_save_session.cc

@@ -111,8 +111,6 @@ int main (int argc, char* argv[])
 
 	AudioEngine::instance()->stop ();
 
-	AudioEngine::destroy ();
-
 	delete test_ui;
 
 	ARDOUR::cleanup ();

libs/ardour/test/profiling/load_session.cc

@@ -44,7 +44,6 @@ int main (int argc, char* argv[])
 	AudioEngine::instance()->remove_session ();
 	delete s;
 	AudioEngine::instance()->stop ();
-	AudioEngine::destroy ();
 	delete test_ui;
 	ARDOUR::cleanup ();
 	return 0;

libs/ardour/test/test_util.cc

@@ -116,7 +116,6 @@ stop_and_destroy_backend ()
 {
 	AudioEngine::instance()->remove_session ();
 	AudioEngine::instance()->stop ();
-	AudioEngine::destroy ();
 }
 
 /** @param dir Session directory.

luasession/luasession.cc

@@ -648,7 +648,6 @@ main (int argc, char** argv)
 	lua = NULL;
 
 	AudioEngine::instance ()->stop ();
-	AudioEngine::destroy ();
 
 	ARDOUR::cleanup ();
 	delete event_loop;