RFR: JDK-8254796: Cleanup pervasive unnecessary parameter

[Jonathan Gibbons]

This is a small cleanup to remove an annoying extra parameter (docEnv) being based around when creating the doclet.

(there's a hint of the old world too, when it is referred to as "rd" in one place, short for "RootDoc": the
corresponding class in the old world ;-) )

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Commit messages:
 - JDK-8254796: Cleanup pervasive unnecessary parameter

Changes: https://git.openjdk.java.net/jdk/pull/672/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=672&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254796
  Stats: 42 lines in 3 files changed: 5 ins; 22 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/672.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/672/head:pull/672

PR: https://git.openjdk.java.net/jdk/pull/672