RFR: 8338662: Shenandoah: Remove excessive ShenandoahVerifier::verify_during_evacuation

[Aleksey Shipilev]

`ShenandoahVerifier::verify_during_evacuation` is a relaxed version of `ShenandoahVerifier::verify_before_evacuation`. In current code, "during" verification is called shortly after "before" check, which really gains us nothing checking-wise, and only really wastes verification time. This is the only "during" verification check we have, all other checks verify things before/after the phases. It makes sense to remove "during evac" verification check for extra debug performance and cleanliness. 

Additional testing:
 - [ ] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
 - [ ] Linux x86_64 server fastdebug, `all` with `-XX:+UseShenandoahGC -XX:+ShenandoahVerify`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20641/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20641&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338662
  Stats: 39 lines in 4 files changed: 4 ins; 34 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20641.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20641/head:pull/20641

PR: https://git.openjdk.org/jdk/pull/20641