RFR: 8321287: Remove unused enum style in Prefetch

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - prefetch-remove

Changes: https://git.openjdk.org/jdk/pull/16955/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16955&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321287
  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16955.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16955/head:pull/16955

PR: https://git.openjdk.org/jdk/pull/16955