RFR: 8345164: Remove residual --enable-preview in FFM tests and benchmarks

[Per Minborg]

This PR proposes to remove redundant `--enable-preview` flags in a test and a benchmark.

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Commit messages:
 - Remove --enable-preview flags

Changes: https://git.openjdk.org/jdk/pull/22512/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22512&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345164
  Stats: 5 lines in 2 files changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/22512.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22512/head:pull/22512

PR: https://git.openjdk.org/jdk/pull/22512