RFR: 8336685: Shenandoah: Remove experimental incremental update mode

[William Kemper]

We've reason to believe that this mode is very rarely used and its maintenance has become a burden for future development.

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Commit messages:
 - Remove last vestiges of incremental update mode
 - Missed test, remove actual IU barrier flag
 - Remove missed iu_barrier usages for C1
 - Update test (all barriers can be enabled now for all modes)
 - WIP: Remove incremental update mode

Changes: https://git.openjdk.org/jdk/pull/20316/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20316&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336685
  Stats: 1696 lines in 69 files changed: 4 ins; 1658 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/20316.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20316/head:pull/20316

PR: https://git.openjdk.org/jdk/pull/20316