RFR: 8335905: CompoundElement API cleanup

[Chen Liang]

`CompoundElement` already inherits `Iterable` and its `forEach(Consumer)`, thus we can remove `Iterable elements()` and `forEachElements(Consumer)`.

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Commit messages:
 - Missed tests
 - 8335905: CompoundElement API cleanup

Changes: https://git.openjdk.org/jdk/pull/20103/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20103&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335905
  Stats: 83 lines in 32 files changed: 2 ins; 11 del; 70 mod
  Patch: https://git.openjdk.org/jdk/pull/20103.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20103/head:pull/20103

PR: https://git.openjdk.org/jdk/pull/20103