Remove deprecated prepare_data_per_node in Trainer

[yhl48]

What does this PR do?

Part of #12521

resolves #8958

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

Remove deprecated prepare_data_per_node in Trainer

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[yhl48]

Not sure if the doc here needs changing? Would appreciate any advice here, thanks.
https://github.com/PyTorchLightning/pytorch-lightning/blob/cf65ca25b0230df89eec77c218c5e928c8141840/pytorch_lightning/core/hooks.py#L364

[rohitgr7]

Not sure if the doc here needs changing? Would appreciate any advice here, thanks.

https://github.com/PyTorchLightning/pytorch-lightning/blob/cf65ca25b0230df89eec77c218c5e928c8141840/pytorch_lightning/core/hooks.py#L364

this should be good:

In DDP ``prepare_data`` can be called in two ways (using ``prepare_data_per_node``):

since there is already a reference for it right below :)