RFR: 8340011: Simplify jdk.internal.classfile.impl.EntryMap

[Claes Redestad]

Minor refactoring to avoid the need to define two anonymous classes that override `EntryMap`. Minor speed-up in interpreter, as well.

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Commit messages:
 - Simplify jdk.internal.classfile.impl.EntryMap

Changes: https://git.openjdk.org/jdk/pull/20966/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20966&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340011
  Stats: 49 lines in 2 files changed: 0 ins; 15 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/20966.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20966/head:pull/20966

PR: https://git.openjdk.org/jdk/pull/20966