RFR: 8374308: ImageBufferCache does not work as intended

[David Beaumont]

Remove ineffective/unused ImageBufferCache class, and simplify callers / remove dead code.

I removed the release methods in the internal classes, but the public ModuleReader API method is still there (the override can go away though since the default implementation also tests for non-null, so removing the override has no risk).

I suspect there are no implementations of ModuleReader that implement release semantics after this change, so perhaps we could relax the documentation around it? Thoughts welcome.

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Commit messages:
 - removing ImageBufferCache and unwinding dead code

Changes: https://git.openjdk.org/jdk/pull/29043/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29043&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374308
  Stats: 54 lines in 3 files changed: 12 ins; 39 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29043.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29043/head:pull/29043

PR: https://git.openjdk.org/jdk/pull/29043