RFR: 8296612: CertAttrSet is useless

[Weijun Wang]

- All `CertAttrSet` child classes now implement `DerEncoder`
- `DerEncoder::derEncode` is renamed to `DerEncoder::encode`, and duplicate methods are removed
- `s.s.x.Extension` now implements `DerEncoder`, so its child classes no need to implement it
- `X509CertImpl::encode(OutputStream out)` removed, used nowhere

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Commit messages:
 - the fix

Changes: https://git.openjdk.org/jdk/pull/11048/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11048&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296612
  Stats: 207 lines in 52 files changed: 4 ins; 134 del; 69 mod
  Patch: https://git.openjdk.org/jdk/pull/11048.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11048/head:pull/11048

PR: https://git.openjdk.org/jdk/pull/11048