Remove deprecated loaded_optimizer_states_dict property

[catalys1]

What does this PR do?

Part of #10312

Remove loaded_optimizer_states_dict property from LightningModule, which was deprecated in v1.4.

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

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[ananthsub]

Thanks for working on this @catalys1 ! Please update the Changelog as well: https://github.com/PyTorchLightning/pytorch-lightning/blob/master/CHANGELOG.md

[carmocca]

I've updated the CHANGELOG for you.

Thank you!