RFR: JDK-8303912 - Clean up JavadocTokenizer

[Jim Laskey]

Previous work had cleaned up the JavacTokenizer, but left JavadocTokenizer alone, mostly due to its perplexing state. This is follow up work to simplify the JavadocTokenizer.

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Commit messages:
 - Clean up JavadocTokenizer

Changes: https://git.openjdk.org/jdk/pull/12953/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12953&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303912
  Stats: 378 lines in 3 files changed: 132 ins; 151 del; 95 mod
  Patch: https://git.openjdk.org/jdk/pull/12953.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12953/head:pull/12953

PR: https://git.openjdk.org/jdk/pull/12953