RFR: 8302709: Remove explicit remembered set verification in G1

[Thomas Schatzl]

Hi all,

  can I have reviews for this imo trivial removal of unnecessary functionality?

So G1 allows explicit remembered set verification via -XX:+VerifyRememberedSets without verifying anything else.

This functionality can be dropped because
- verification via VerifyBefore/During/AfterGC does the same verification as well
- the far longest part of VerifyBefore/During/AfterGC is remembered set/object verification (you can't really do remembered set verification without object verification)
- since you can't really do remembered set verification without object verification, this is just duplicate work (and code)
- for VerifyBefore/During/AfterGC we have quite extensive ways to do that verification only in certain situations, so this extra remembered set verification is mostly just in the way
- explicit remembered set verification is single threaded, while others is multi-threaded, so much slower then the latter too
- the split between remembered set/object verification forces some awkward code sharing that makes the verification code somewhat confusing

Given all that I suggest to remove the explicit remembered set verification there. 

Note that this change does not clean up the mentioned code sharing, I'll do this in a follow up.

This change depends on PR#12511.

Testing: local compilation, gha

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Depends on: https://git.openjdk.org/jdk/pull/12511

Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/12611/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12611&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302709
  Stats: 62 lines in 3 files changed: 0 ins; 62 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12611.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12611/head:pull/12611

PR: https://git.openjdk.org/jdk/pull/12611