Removed the deprecated datamodule_checkpointhooks

[NeoKish]

What does this PR do?

Remove deprecate API

Part of #14841
resolves #11887

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[awaelchli]

@NeoKish Thanks for the PR. The actual removal of what was deprecated was missing. It probably wasn't very clear of what should have been done, so I figured I push a commit to help you out, hope that's ok. Waiting for tests to pass.

Nice work!