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/alsa/alsa_audiobackend.cc

@@ -1171,12 +1171,6 @@ AlsaAudioBackend::my_name () const
 	return _instance_name;
 }
 
-bool
-AlsaAudioBackend::available () const
-{
-	return _run && _active;
-}
-
 uint32_t
 AlsaAudioBackend::port_name_size () const
 {

libs/backends/alsa/alsa_audiobackend.h

@@ -270,7 +270,6 @@ class AlsaAudioBackend : 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&);