RFR: 8320447: Remove obsolete `LintCategory.hidden`

[Jonathan Gibbons]

On Tue, 21 Nov 2023 08:41:00 GMT, Christian Stein <cstein at openjdk.org> wrote:

> Please review this change that removes the `LintCategory.hidden` member.
> 
> `LintCategory.hidden` is an obsolete member with no uses. It was used for a custom "internal" category, "sunapi".

Marked as reviewed by jjg (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16755#pullrequestreview-1742318596