Remove the deprecated LightningDataModule.val_transforms

[rschireman]

What does this PR do?

Fixes part of #12521 by removing the deprecated LightningDataModule.val_transforms

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

Removes deprecated LightningDataModule.val_transforms

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[akihironitta]

Hi @rschireman! Thank you for working on this! One of the CI jobs seems to have failed as seen in https://results.pre-commit.ci/run/github/178626720/1650559831.ycbj-8PdT7WOECLh8cRgcQ. Could you remove unused import statements to fix the CI failure?

[rschireman]

@akihironitta no problem, PyTorch Lightning is awesome so it's my pleasure! I just removed the unused imports and pushed changes!

[akihironitta]

@rschireman It seems it has passed now :) Thanks again for your action! ❤️