RFR: Use relevant variables in log messages, remove unused import

[William Kemper]

Addresses feedback in PR#220.

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Commit messages:
 - Use relevant variables in log messages, remove unused import

Changes: https://git.openjdk.org/shenandoah/pull/222/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=222&range=00
  Stats: 35 lines in 1 file changed: 10 ins; 23 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/222.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/222/head:pull/222

PR: https://git.openjdk.org/shenandoah/pull/222