RFR: 8373039: Remove Incorrect Asserts in shenandoahScanRemembered [v2]

[Ben Taylor]

> The `Klass->is_valid` asserts in this file do not hold the required `ClassLoaderDataGraph_lock` and can cause a crash.
> 
> A similar issue was seen in https://bugs.openjdk.org/browse/JDK-8372566
> 
> This change passes all tests in `TEST=hotspot_gc_shenandoah` with a fastdebug build

Ben Taylor has updated the pull request incrementally with one additional commit since the last revision:

  Fix up comment and remove additional assert from previous commit

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28642/files
  - new: https://git.openjdk.org/jdk/pull/28642/files/03456d6f..eec662f6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28642&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28642&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28642.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28642/head:pull/28642

PR: https://git.openjdk.org/jdk/pull/28642