RFR: 8309159: Some minor comment and code cleanup in jdk/com/sun/jdi/PopFramesTest.java

[Chris Plummer]

PopFramesTest.java is a new test. It could use a couple of minor code cleanups, and there is one unused local variable that can be removed.

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Commit messages:
 - Some minor comment and code cleanup

Changes: https://git.openjdk.org/jdk/pull/14237/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14237&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309159
  Stats: 5 lines in 1 file changed: 1 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14237.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14237/head:pull/14237

PR: https://git.openjdk.org/jdk/pull/14237