Remove deprecated property is_using_torchelastic from AcceleratorConnector

[daniellepintz]

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[rohitgr7]

aren't all deprecations removed in a single PR?? there is 1 more.

[awaelchli]

@rohitgr7 I can't see any more usages that are from the accelerator connector. All the rest are from the TorchElasticEnvironment

[rohitgr7]

@awaelchli I meant the other deprecation. https://github.com/PyTorchLightning/pytorch-lightning/blob/131176b9f5e544a1133240a46dca1960308b7849/pytorch_lightning/trainer/trainer.py#L230

[awaelchli]

there is already a PR for it but its blocked: #9246
normally deprecated api gets removed in individual PRs