_scheduler/queue/trackqueue.py: Remove unused import

The `SourceError` import was never used

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

buildstream/_scheduler/queues/trackqueue.py

@@ -20,7 +20,6 @@
 
 # BuildStream toplevel imports
 from ...plugin import _plugin_lookup
-from ... import SourceError
 
 # Local imports
 from . import Queue, QueueStatus