Remove deprecated TestTubeLogger

[stanbiryukov]

What does this PR do?

Fixes #12858, which is subset of #12521

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

Removes from pytorch_lightning.loggers import TestTubeLogger

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[akihironitta]

Hi @stanbiryukov Thank you for your work! The CI jobs are failing due to the import statement here. Would you mind removing this import statement, too?

  File "/home/runner/work/pytorch-lightning/pytorch-lightning/pytorch_lightning/loggers/__init__.py", line 28, in <module>
    from pytorch_lightning.loggers.test_tube import _TESTTUBE_AVAILABLE, TestTubeLogger  # noqa: F401
ModuleNotFoundError: No module named 'pytorch_lightning.loggers.test_tube'

[stanbiryukov]

Hi @stanbiryukov Thank you for your work! The CI jobs are failing due to the import statement here. Would you mind removing this import statement, too?

  File "/home/runner/work/pytorch-lightning/pytorch-lightning/pytorch_lightning/loggers/__init__.py", line 28, in <module>
    from pytorch_lightning.loggers.test_tube import _TESTTUBE_AVAILABLE, TestTubeLogger  # noqa: F401
ModuleNotFoundError: No module named 'pytorch_lightning.loggers.test_tube'

Thanks, @akihironitta. Good point on the __init__.py file. I also updated tests/loggers/test_all.py so those now pass without test_tube.

[akihironitta]

Once an entry is added to the changelog, LGTM!

[stanbiryukov]

Once an entry is added to the changelog, LGTM!

Thanks @akihironitta - added the removal entry to CHANGELOG.md