RFR: 8342679: Cleanup PreviewFeature.Feature

[Aggelos Biboudis]

After doing a build + bootcycle build confirms that these constants can be removed.

-------------

Commit messages:
 - 8342679: Cleanup PreviewFeature.Feature

Changes: https://git.openjdk.org/jdk/pull/21605/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21605&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342679
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21605.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21605/head:pull/21605

PR: https://git.openjdk.org/jdk/pull/21605