RFR: 8338473: GenShen: Cleanup access levels, whitespace, typos and unused code

[William Kemper]

This change cleans up access levels, uses keyword `override` instead of `virtual`. Fixes whitespace and typos. Removes unused code an unnecessary assert.

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Commit messages:
 - Remove unnecessary assert
 - Remove unused code
 - Fix typos and whitespace
 - Clean up access levels, use override instead of virtual

Changes: https://git.openjdk.org/shenandoah/pull/471/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=471&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338473
  Stats: 58 lines in 9 files changed: 3 ins; 45 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah/pull/471.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/471/head:pull/471

PR: https://git.openjdk.org/shenandoah/pull/471