RFR: 8286475: Drop --enable-preview from instanceof pattern matching 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 instanceof pattern matching tests, since instanceof pattern matching have been graduated from preview in JDK 16. 

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/8628/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8628&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286475
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8628.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8628/head:pull/8628

PR: https://git.openjdk.java.net/jdk/pull/8628