RFR: 8365197: javax.imageio.stream MemoryCache based streams no longer need a disposer.

[Phil Race]

The disposer for MemoryCacheImageInputStream isn't needed. All it does is clear an ArrayList of byte[]
GC can take care of that.

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Commit messages:
 - 8365197

Changes: https://git.openjdk.org/jdk/pull/26933/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26933&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365197
  Stats: 28 lines in 1 file changed: 1 ins; 27 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26933.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26933/head:pull/26933

PR: https://git.openjdk.org/jdk/pull/26933