Since 62fc1d3c2e, Delivery buffers were flushed twice.
Once by copy_to_outputs() and again later by
Delivery::flush_buffers. This resulted in duplicate events
during export (see 576840c09e, MIDI buffers are not cleared
after flush to allow export processing to grab the data from
the port-buffers).
The workaround in 62fc1d3c2e is only relevant for ClickIO,
other Deliveries (Send is a Delivery) are explicitly flushed
by Route::flush_processor_buffers_locked.
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