Remove AcceleratorConnector.has_ipu

[DuYicong515]

What does this PR do?

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

AcceleratorConnector.has_ipu has no usage internally and was not meant to be a publish property.
It's removed in favor of using isinstance(self.accelerator, IPUAccelerator) and isinstance(self.strategy, IPUStrategy) directly

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

None

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