Fix duplicate MIDI Clock messages

Notably during realtime export an explicit flush of async
MIDI ports will duplicate port buffer content.

AsyncMIDIPort::cycle_end () calls `flush_output_fifo`,
and `flush_buffers` is called from PortManager::cycle_end.

libs/ardour/ticker.cc

@@ -211,7 +211,6 @@ MidiClockTicker::tick (samplepos_t start_sample, samplepos_t end_sample, pframes
 	_transport_pos = end_sample;
 
 out:
-	_midi_port->flush_buffers (n_samples);
 	_midi_port->cycle_end (n_samples);
 }