wgsl: Cleanup cruft related to quantizeToF16

[zoddicus]

This is only defined for f32, so doesn't really need to be defined in the other classes that will never implement it.

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

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[dneto0]

LGTM.

I saw there was an assertion for code that would have invoked a test for quantizeToF16 for a non-f32 based argument. But that seems to not be exercised.
I concluded this was fine because the assert itself was never triggered (otherwise the TS copmijle would fail). It would be some other test or MR that would check for a validation failure. So, after all, this is fine.