RFR: 8232066: Remove outdated code/methods from PKIX implementation

[Sean Mullan]

Please review this fix to remove the Validator.VAR_PLUGIN_CODE_SIGNING variant and related code that is no longer needed in the PKIX implementation.

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Commit messages:
 - 8232066: Remove outdated code/methods from PKIX implementation

Changes: https://git.openjdk.java.net/jdk/pull/4934/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4934&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8232066
  Stats: 182 lines in 7 files changed: 6 ins; 167 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4934.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4934/head:pull/4934

PR: https://git.openjdk.java.net/jdk/pull/4934