Integrated: Fix signature attribute generation for null restrictions

[Vicente Romero]

On Fri, 19 Dec 2025 18:21:32 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> The code in `Types` is already correctly handling null restrictions.
> There's only a minor problem with the flag to enable null restriction not being passed recursively in nested call (e.g. for method signatures).
> 
> Since I was there, I also dropped signature generation code for type variable declarations.

lgtm

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Marked as reviewed by vromero (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1848#pullrequestreview-3599593952