RFR: 8286515: JFR: Remove SimpleStringIdPool class

[Erik Gahlin]

Could I have review of a fix that removes an unnecessary indirection when using the string pool.

Testing: jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Move non-final field
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/8638/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8638&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286515
  Stats: 96 lines in 1 file changed: 24 ins; 39 del; 33 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8638.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8638/head:pull/8638

PR: https://git.openjdk.java.net/jdk/pull/8638