RFR: 8295879: JFR: Remove warnings in the jdk.jfr module

[Erik Gahlin]

Hi,

Could I have a review of PR that removes some warnings in the jdk.jfr module. 

Testing: jdk/jdk/jfr + tier1

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.org/jdk/pull/10850/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10850&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295879
  Stats: 69 lines in 21 files changed: 0 ins; 69 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10850.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10850/head:pull/10850

PR: https://git.openjdk.org/jdk/pull/10850