RFR: 8336685: Shenandoah: Remove experimental incremental update mode [v4]

[William Kemper]

> We've reason to believe that this mode is very rarely used and its maintenance has become a burden for future development.
> 
> ## Testing
> * hotspot_gc_shenandoah
> * dacapo
> * diluvian
> * extremem
> * hyperalloc
> * specjbb2015
> * specjvm2008

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge remote-tracking branch 'openjdk/master' into remove-incremental-update-mode
 - Simplify final mark now that incremental update mode is removed
 - Remove unintentional new line
 - 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:
  - all: https://git.openjdk.org/jdk/pull/20316/files
  - new: https://git.openjdk.org/jdk/pull/20316/files/79ceade3..287b6187

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20316&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20316&range=02-03

  Stats: 10966 lines in 271 files changed: 7634 ins; 1955 del; 1377 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