Revert "Ensure disk buffer refills happen in the right thread"

This reverts commit 96ebac646d.

There are some valid cases where refill is called from the GUI
thread. e.g adding tracks, or adding channels to an existing track.

libs/ardour/disk_reader.cc

@@ -1158,8 +1158,6 @@ DiskReader::refill_audio (Sample* sum_buffer, Sample* mixdown_buffer, float* gai
 		return 0;
 	}
 
-	assert (0 == strcmp ("butler", pthread_name ()) || _session.exporting ());
-
 	int32_t                        ret = 0;
 	samplecnt_t                    zero_fill;
 	uint32_t                       chan_n;