Fix MIDI export and post-export transport issues

Bug was introduced in 128a45954c, declick-amp gain was
overridden, but declick not cleared. For some reason this did
not affect audio-only exports nor all session exports.

libs/ardour/disk_reader.cc

@@ -499,7 +499,10 @@ DiskReader::run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_samp
 bool
 DiskReader::declick_in_progress () const
 {
-	return _session.cfg()->get_use_transport_fades() && (_declick_amp.gain() != 0); // declick-out
+	if (!_session.cfg()->get_use_transport_fades () || (_session.exporting () && ! _session.realtime_export ())) {
+		return false;
+	}
+	return (_declick_amp.gain() != 0); // declick-out
 }
 
 bool