RFR: 8370441: Remove unnecessary/confusing null check in Verifier::verify()

[Paul Hübner]

[JDK-8059924](https://bugs.openjdk.org/browse/JDK-8059924) introduced a nullcheck for a klass' Java mirror which runs during verification. However, in the modern VM, during verification time, the mirror is guaranteed to exist making the check unnecessary and misleading when reading the code. While implementing [JDK-8366488](https://bugs.openjdk.org/browse/JDK-8366488) it caused for quite some confusion. With this PR it is my intention to make sure that nobody gets side-tracked by this impossible condition.

For an insight into previous discussion on this topic, please see the associated JBS issue.

Testing: tiers 1-5 on Linux x64, AArch64, macOS x64, AArch64, Windows x64.

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Commit messages:
 - Turn check into an assert.

Changes: https://git.openjdk.org/jdk/pull/29314/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29314&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370441
  Stats: 12 lines in 1 file changed: 0 ins; 11 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29314.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29314/head:pull/29314

PR: https://git.openjdk.org/jdk/pull/29314