feat: remove is valid token endpoint

[Vafilor]

What this PR does:

Which issue(s) this PR fixes:

Fixes onepanelio/core#

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Checklist

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• I have added/updated relevant unit tests
• I have added/updated relevant documentation

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• I accept to release these changes under the Apache 2.0 License

[rushtehrani]

@Vafilor we need to revert this to maintain backwards compatibility with older CVAT Workspaces. We need to instead mark this as deprecated by adding an option:

option deprecated = true;