RFR: 8336685: Shenandoah: Remove experimental incremental update mode [v3]

[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 incrementally with one additional commit since the last revision:

  Simplify final mark now that incremental update mode is removed

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20316/files
  - new: https://git.openjdk.org/jdk/pull/20316/files/ec2d6b64..79ceade3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20316&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20316&range=01-02

  Stats: 13 lines in 1 file changed: 0 ins; 9 del; 4 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