RFR: 8354944: Remove unnecessary PartiallyOrderedSet.nodes

[Andrey Turbanov]

We can use `javax.imageio.spi.PartiallyOrderedSet#poNodes` directly

-------------

Commit messages:
 - [PATCH] Remove unnecessary PartiallyOrderedSet.nodes

Changes: https://git.openjdk.org/jdk/pull/24699/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24699&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354944
  Stats: 9 lines in 1 file changed: 0 ins; 4 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/24699.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24699/head:pull/24699

PR: https://git.openjdk.org/jdk/pull/24699