remove the `declarationKind option`

[n1ru4l]

In my opinion, the option does not provide any benefit over just using type, by removing it we remove complexity.

This would be a breaking change to consider for the next major.

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Related:

• @oneOf support broken with interfaces #7938

[dominikx96]

@n1ru4l this option gives me the ability to generate classes (necessary for eg. swagger responses).