RFR: 8270171: Shenandoah: Cleanup TestStringDedup and TestStringDedupStress tests

[Zhengyu Gu]

Please review this trivial patch to cleanup TestStringDedup and TestStringDedupStress tests, removes unused Unsafe usages.

Test:
- [x] Both tests still pass

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Commit messages:
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/4742/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4742&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270171
  Stats: 20 lines in 2 files changed: 0 ins; 19 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4742.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4742/head:pull/4742

PR: https://git.openjdk.java.net/jdk/pull/4742