Remove tf_to_hls

[vloncar]

Description

This PR removes tf_to_hls. The tf_to_hls converter is outdated, unused, not exposed through the main API and based on old approach that likely won't work anymore. If we decide to support lower-level tensorflow API alongside Keras, this approach would be different anyway, so there's no reason to keep this around.

A question to be addressed: this converter introduced BiasAdd as a temporary layer and the associated optimizer. If ONNX and PyTorch converters don't plan to use it, it should be removed in this PR as well.

Type of change

• [x] Other (Specify) - Cleanup

Tests

Nothing to test.

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have installed and run pre-commit on the files I edited or added.
• [x] I have added tests that prove my fix is effective or that my feature works.

[jmitrevs]

I think the pytest failure is transient. The test succeed on my computer.