Remove dsp_has_midi_*() in favor of dsp_ioconfig()

_has_midi_*put members of LuaProc will be set according to the actual
configuration chosen, for configure_io() and run() to use.

scripts/midifilter.lua

@@ -8,12 +8,9 @@ ardour {
 }
 
 function dsp_ioconfig ()
-	return { { audio_in = 0, audio_out = 0}, }
+	return { { midi_in = 1, midi_out = 1, audio_in = 0, audio_out = 0}, }
 end
 
-function dsp_has_midi_input () return true end
-function dsp_has_midi_output () return true end
-
 function dsp_run (_, _, n_samples)
 	assert (type(midiin) == "table")
 	assert (type(midiout) == "table")