RFR: 8320447: Remove obsolete `LintCategory.hidden`

[Christian Stein]

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".

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Commit messages:
 - Remove obsolete `LintCategory.hidden`

Changes: https://git.openjdk.org/jdk/pull/16755/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16755&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320447
  Stats: 7 lines in 1 file changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16755.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16755/head:pull/16755

PR: https://git.openjdk.org/jdk/pull/16755