RFR: 8365198: Remove unnecessary mention of finalize in ImageIO reader/writer docs

[Phil Race]

Image I/O's ImageReader and ImageWriter have never had finalize() methods, yet have a gratuitous mention of finalize(). This should be removed. Simple doc only change

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Commit messages:
 - 8365198

Changes: https://git.openjdk.org/jdk/pull/26753/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26753&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365198
  Stats: 4 lines in 2 files changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26753.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26753/head:pull/26753

PR: https://git.openjdk.org/jdk/pull/26753