fix endless messages about MIDI events being dropped.

track-owned MIDI port buffers (at the Ardour level) get ::flush_buffers() called twice, once by Delivery::flush_buffers() which is called
at the end of MidiTrack::rol() to push data out in graph order, and then finally as a last-chance effort in PortManager::cycle_end().

This should not cause a repeated attempt to write the same data, but it was. Fixed by marking the buffer empty once its data has
been flushed into a backend port buffer.