RFR: JDK-8301636: Minor cleanup in CommentHelper and DocPretty

[Jonathan Gibbons]

Please review some simple cleanup suggested by IntelliJ IDEA that would be too much of a distraction in an otherwise unrelated PR.

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Commit messages:
 - JDK-8301636: Minor cleanup in CommentHelper and DocPretty

Changes: https://git.openjdk.org/jdk/pull/12375/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12375&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301636
  Stats: 79 lines in 2 files changed: 0 ins; 52 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/12375.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12375/head:pull/12375

PR: https://git.openjdk.org/jdk/pull/12375