Remove now-unused channel errors (#3713)

The final use of channel errors was removed in PR #3690

This is part of #3515 channel removal project

# Changed Behaviour

Any user catching these errors explicitly will now get an error about
the exceptions being undefined. Because these don't exist any more, it
should be safe to remove those catches.

## Type of change

- Code maintenance/cleanup

Author: benclifford

docs/reference.rst

@@ -169,8 +169,6 @@ Exceptions
     parsl.providers.errors.ScaleOutFailed
     parsl.providers.errors.SchedulerMissingArgs
     parsl.providers.errors.ScriptPathError
-    parsl.channels.errors.ChannelError
-    parsl.channels.errors.FileCopyException
     parsl.executors.high_throughput.errors.WorkerLost
     parsl.executors.high_throughput.interchange.ManagerLost
     parsl.serialize.errors.DeserializationError