breaking: remove legacy TensorFlowModel and TensorFlowPredictor classes

[laurenyu]

Issue #, if available:
#1462

Description of changes:
This change also removes the associated serialization/deserialization code used by TensorFlowPredictor and the locally copied TFS APIs.

Due to sagemaker.tensorflow.serving.Model not accepting model_server_workers, I ended up having to keep the TF-specific transformer() implementation, but I'd accidentally deleted the unit tests in #1510 (must've messed up my git branches), so that's why those are getting readded here.

I'll move the sagemaker.tensorflow.serving classes to match the old class names/paths but I didn't want to do it in this PR in case it made the diff look unreadable.

Testing done:

• tox -e pylint,flake8,black-format --parallel all
• pytest tests/unit/test_tf*
• pytest tests/integ/test_tf*

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• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have passed the region in to any/all clients that I've initialized as part of this change.
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[metrizable]

looks good to me