RFR: 8232066: Remove outdated code/methods from PKIX implementation [v2]

[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.

Sean Mullan has updated the pull request incrementally with one additional commit since the last revision:

  Remove NPE check in Validator.getInstance. Update ConstructorTest not to test for this case anymore.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4934/files
  - new: https://git.openjdk.java.net/jdk/pull/4934/files/52be3dd2..b388bb4b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4934&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4934&range=00-01

  Stats: 26 lines in 2 files changed: 0 ins; 21 del; 5 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