RFR: 7235: Some writer cleanup

[Marcus Hirt]

Just getting rid of a few annoying warnings. I'll add an issue once JBS is up again.

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Commit messages:
 - Some writer cleanup

Changes: https://git.openjdk.java.net/jmc/pull/237/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=237&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7235
  Stats: 5 lines in 3 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/237.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/237/head:pull/237

PR: https://git.openjdk.java.net/jmc/pull/237