Remove ambiguous API implementation

* PortEngine::available() implementation
* AudioEngine::connected() wrapper

Eventually we may re-introduce PortEngine::available along
with a libardour internal port-engine.

libs/backends/dummy/dummy_audiobackend.cc

@@ -635,12 +635,6 @@ DummyAudioBackend::my_name () const
 	return _instance_name;
 }
 
-bool
-DummyAudioBackend::available () const
-{
-	return _running;
-}
-
 uint32_t
 DummyAudioBackend::port_name_size () const
 {

libs/backends/dummy/dummy_audiobackend.h

@@ -323,7 +323,6 @@ class DummyAudioBackend : public AudioBackend {
 
 		void* private_handle () const;
 		const std::string& my_name () const;
-		bool available () const;
 		uint32_t port_name_size () const;
 
 		int         set_port_name (PortHandle, const std::string&);