RFR: 8373039: Remove Incorrect Asserts in shenandoahScanRemembered

[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

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Commit messages:
 - 8373039: Remove Incorrect Asserts in shenandoahScanRemembered

Changes: https://git.openjdk.org/jdk/pull/28642/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28642&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373039
  Stats: 9 lines in 1 file changed: 0 ins; 9 del; 0 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