Remove deprecated checkpoint_callback flag in Trainer

[shenoynikhil]

Part of #12521

What does this PR do?

This PR removes the checkpoint_callback flag from the Trainer Class. Following edits happen,

• pytorch_lightning/trainer/trainer.py
• pytorch_lightning/trainer/connectors/callback_connector.py
• CHANGELOG
• removed test from tests/deprecated_api/test_remove_1-7.py
• updated line from simple_classif_training.py

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

- Trainer(checkpoint_callback=False)
+ Trainer(enable_checkpointing=False)

resolves #9754

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Make sure you had fun coding 🙃

[shenoynikhil]

Dependence in Other Important Repostories

Based on @kaushikb11, I did a search on grep.app and found multiple important examples from

• huggingface [Example 1] [Example 2]
• NVIDIA [Example 1]
  Is this something we still plan to do?

[shenoynikhil]

@akihironitta This is my first contribution to PL, let me know if I have missed something obvious :).

[akihironitta]

Thanks for working on this! LGTM!