Drop: legacy_file_upload endpoints

[psrok1]

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• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
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What is the current behaviour?

That's another feature that was deprecated since initial release. Earlier versions of MWDB used /api/file/<parent> endpoint to upload and "attach" a new object to the existing parent. In case of no parent, /api/file/root was used instead.

It was pretty obscure way of uploading objects so it was replaced with POST /api/<object_type> endpoint, but previous endpoint was left for compatibility reasons

What is the new behaviour?

Legacy upload endpoints are removed.