Drop unused Deserialize

[xzfc]

A lot of structs has a Deserialize derive despite we never deserialize them.

It is inconvenient for the following reasons:

• When adding new fields to structures, we need to provide validation or reason about "what if an user pass an unexpected data?". If a struct is not Deserialize, a developer could search and review all the places in the code base where this specific value originates. If it's Deserialize, this means that data could be arbitrary. E.g., my histogram bucket merging code works fine for data being generated by my code, but it's not true for arbitrary data.
• Double the work if we decide to add a custom impl Serialize for some field since then we have to implement a custom Deserialize too.

This PR drops unused Deserialize impls. Precisely, the following things removed:

• #[derive(Deserialize)], obviously.
• #[serde(default)] for non-deserializable structs.
• #[derive(Validate)] and #[validate] in one place.

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New Feature Submissions:

1. Does your submission pass tests?
2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
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Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
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[xzfc]

It's output-only structure and this field is never skipped, so the new schema is more accurate.