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

Hi, Could I have a review of PR that removes some warnings in the jdk.jfr module. Testing: jdk/jdk/jfr + tier1 Thanks Erik ------------- 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