[lworld] RFR: 8371357: [lworld] Remove EnableValhalla

[Paul Hübner]

Hi all,

This removes the `EnableValhalla` in favour of the `--enable-preview` flag. Concretely:
* I've replaced most of the `EnableValhalla` checks with `Arguments::is_valhalla_enabled()`. 
* Some checks were redundant and could be removed entirely.
* I've made the `EnableValhalla` flag obsolete.
* Some tests had to be updated.

This greatly changes the semantics of tests. I've refined some test groups to make it easier.

Testing: tiers 1-4.

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Commit messages:
 - Reviewer comments.
 - Copyright.
 - Revert "Don't synchronize on Double."
 - Accidentally changed too many enable_preview.
 - Make heap dump test require debug VM.
 - Change to arguments.
 - Use is_valhalla_enabled.
 - Remove unused file.
 - Reset problem lists.
 - Explode JDK groups and problemlist tests.
 - ... and 9 more: https://git.openjdk.org/valhalla/compare/c6706524...e8e771a9

Changes: https://git.openjdk.org/valhalla/pull/1759/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1759&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371357
  Stats: 157 lines in 55 files changed: 59 ins; 13 del; 85 mod
  Patch: https://git.openjdk.org/valhalla/pull/1759.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1759/head:pull/1759

PR: https://git.openjdk.org/valhalla/pull/1759