RFR: 8336685: Shenandoah: Remove experimental incremental update mode

[William Kemper]

Not a clean backport.

# Testing
GHA, Dacapo, Extremem, Hyperalloc, Specjbb2015, Specjvm2008, Diluvian (with and without stress flags).

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

Commit messages:
 - 8336685: Shenandoah: Remove experimental incremental update mode

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/160/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=160&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336685
  Stats: 1709 lines in 70 files changed: 4 ins; 1668 del; 37 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/160.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/160/head:pull/160

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/160