Remove AcceleratorConnector.devices

[DuYicong515]

What does this PR do?

Part of #11449: remove unused properties in accelerator_connector: devices

AcceleratorConnector.devices has no usage internally and was not meant to be a publish property.
It's removed in favor of using Trainer.num_devices directly

Does your PR introduce any breaking changes? If yes, please list them.

N/A

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