audio: Remove unused constant

Removes the never used constant AUDIO_INTERFACE which is defined to
"org.bluez.Audio".
diff --git a/profiles/audio/device.c b/profiles/audio/device.c
index 907221d..b9d1dde 100644
--- a/profiles/audio/device.c
+++ b/profiles/audio/device.c
@@ -56,8 +56,6 @@
 #include "sink.h"
 #include "source.h"
 
-#define AUDIO_INTERFACE "org.bluez.Audio"
-
 #define CONTROL_CONNECT_TIMEOUT 2
 #define AVDTP_CONNECT_TIMEOUT 1
 #define AVDTP_CONNECT_TIMEOUT_BOOST 1