[lworld] RFR: 8371357: [lworld] Remove EnableValhalla [v2]

[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.

Paul Hübner has updated the pull request incrementally with two additional commits since the last revision:

 - Remove wrong comment.
 - Fix imports.

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1759/files
  - new: https://git.openjdk.org/valhalla/pull/1759/files/e8e771a9..71f120ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1759&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1759&range=00-01

  Stats: 5 lines in 3 files changed: 2 ins; 3 del; 0 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