chore(config): remove configurable derived annotations

[pront]

Summary

Motivation

#[configurable(derived)] only bypassed the macro-time field-description check. It was not emitted into metadata or consulted during schema generation, which already falls back to the field type's documentation.

Changes

• remove the field-description compile-time requirement and the derived macro option
• remove all derived annotations from configuration fields
• simplify the now-unused virtual-newtype parsing plumbing

References

Closes #15271.

Vector configuration

Not applicable. The generated schema and component documentation remain unchanged.

How did you test this PR?

• generated component documentation from clean master and this branch; tracked output is identical
• normalized generated configuration schemas from clean master and this branch are identical
• cargo test -p vector-config (56 passed)
• cargo fmt --all

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment in the changelog.d directory.
• No. A changelog fragment is not required.