RFR: 8286474: Drop --enable-preview from Sealed Classes related tests

[Aleksey Shipilev]

There are plenty of tests failing on many architectures due to `--enable-preview` VM code introduced by Loom. This improvement eliminates some of the redundant `--enable-preview` clauses from the Sealed Classes tests, since Sealed Classes have been graduated from preview in JDK 17.

Additional testing:
 - [x] Linux x86_64 fastdebug, affected tests still pass
 - [x] Linux x86_32 fastdebug, affected tests start to pass

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

Commit messages:
 - Some

Changes: https://git.openjdk.java.net/jdk/pull/8627/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8627&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286474
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8627.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8627/head:pull/8627

PR: https://git.openjdk.java.net/jdk/pull/8627