RFR: Replace 'HEY!' comments with TODO and remove unnecessary usages

[William Kemper]

Replace idiosyncratic `HEY!` comments with `TODO` and remove it from places where it is no longer necessary. No code changes here, just changes in comments.

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Commit messages:
 - Replace 'HEY!' comments with TODO and remove unnecessary usages

Changes: https://git.openjdk.java.net/shenandoah/pull/87/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=87&range=00
  Stats: 14 lines in 6 files changed: 0 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/87.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/87/head:pull/87

PR: https://git.openjdk.java.net/shenandoah/pull/87