RFR: 8345346: Shenandoah: Description of ShenandoahGCMode still refers to incremental update mode

[William Kemper]

The incremental update mode has been removed and is no longer supported.

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Commit messages:
 - Remove reference to incremental update mode

Changes: https://git.openjdk.org/jdk/pull/22502/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22502&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345346
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22502.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22502/head:pull/22502

PR: https://git.openjdk.org/jdk/pull/22502