RFR: 8279682: JFR: Remove dead code

[Erik Gahlin]

Hi,

Could I have a review of removal of some code. I also made a class final, so unused fields are easier to spot. 

Testing: jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/7006/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7006&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279682
  Stats: 61 lines in 6 files changed: 0 ins; 59 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7006.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7006/head:pull/7006

PR: https://git.openjdk.java.net/jdk/pull/7006