alsa driver: remove midi-driver long description

The parameter long description is used through the controlapi/dbus.
Now that jack_control displays the constraints, the dither parameter
long description servers no purpose.

linux/alsa/JackAlsaDriver.cpp

@@ -743,11 +743,8 @@ SERVER_EXPORT const jack_driver_desc_t* driver_get_descriptor ()
         jack_constraint_compose_enum_str(
             JACK_CONSTRAINT_FLAG_STRICT | JACK_CONSTRAINT_FLAG_FAKE_VALUE,
             midi_constraint_descr_array),
-        "ALSA device name",
-        "ALSA MIDI driver:\n"
-        " none - no MIDI driver\n"
-        " seq - ALSA Sequencer driver\n"
-        " raw - ALSA RawMIDI driver\n");
+        "ALSA MIDI driver",
+        NULL);
 
     return desc;
 }