RFR: 8334031: Generated JfrNativeSettings seems off

[Robert Toyonaga]

This PR changes GenerateJfrFiles.java so that the generated `JfrNativeSettings` union does not include JFR structs.`JfrNativeSettings` is meant to hold the settings for JFR events, but previously also included JFR structs such as MetaspaceSizes, StackFrame, CopyFailed, G1EvacuationStatistics, ObjectSpace, VirtualSpace. These are not events, but instead are JFR `Type`s, and so do not have settings such as stacktraces or thresholds.

The inclusion of JFR structs in `JfrNativeSettings` was problematic because it could cause a displacement between event ID and`JfrNativeSettings` array index (each index is meant to correspond with a specific event ID).

Testing:
- jdk/jdk/jfr
- tier1

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Commit messages:
 - Don't include JFR structs in the list of JFR events in generated files.

Changes: https://git.openjdk.org/jdk/pull/19891/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19891&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334031
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19891.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19891/head:pull/19891

PR: https://git.openjdk.org/jdk/pull/19891