[jdk16u] RFR: 8260048: Shenandoah: ShenandoahMarkingContext asserts are unnecessary

[Aleksey Shipilev]

This trivial backport simplifies the fastpath in fastdebug builds.

Additional testing:
 - [x] hotspot_gc_shenandoah

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

Commit messages:
 - Backport 5940287b9fa494cf8ddff634a698b0f4891b6f7b

Changes: https://git.openjdk.java.net/jdk16u/pull/53/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=53&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260048
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/53.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/53/head:pull/53

PR: https://git.openjdk.java.net/jdk16u/pull/53