RFR: 8296612: CertAttrSet is useless [v2]

[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

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  overrides

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11048/files
  - new: https://git.openjdk.org/jdk/pull/11048/files/889e728e..8ae5fcd0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11048&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11048&range=00-01

  Stats: 7 lines in 7 files changed: 7 ins; 0 del; 0 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