RFR: 8284447: Remove the unused NestedClassWriter interface

[Pavel Rappo]

The `NestedClassWriter` interface seems to have never been used. I delete it and reuse the freed name for the `NestedClassWriterImpl` class. If I don't rename `NestedClassWriterImpl` to `NestedClassWriter`, it might confuse future maintainers. Indeed, the "Impl" suffix on a class' name typically implies that there is an interface which that class implements.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/8129/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8129&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284447
  Stats: 40 lines in 5 files changed: 0 ins; 33 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8129.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8129/head:pull/8129

PR: https://git.openjdk.java.net/jdk/pull/8129