RFR: 8329749: UseNeon flag is unused

[Tobias Hartmann]

After [JDK-8328264](https://bugs.openjdk.org/browse/JDK-8328264), the UseNeon flag is only used by JVMCI code and @dougxc confirmed that it can be removed.

Thanks,
Tobias

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Commit messages:
 - 8329749: UseNeon flag is unused

Changes: https://git.openjdk.org/jdk/pull/18648/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18648&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329749
  Stats: 8 lines in 3 files changed: 0 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18648.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18648/head:pull/18648

PR: https://git.openjdk.org/jdk/pull/18648