RFR: JDK-8263050: move HtmlDocletWriter.verticalSeparator to IndexWriter

[Jonathan Gibbons]

Please review a tiny change to move a small utility method from a superclass into the one subclass where it is actually used.

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Commit messages:
 - JDK-8263050: move HtmlDocletWriter.verticalSeparator to IndexWriter

Changes: https://git.openjdk.java.net/jdk/pull/2837/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2837&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263050
  Stats: 8 lines in 2 files changed: 4 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2837.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2837/head:pull/2837

PR: https://git.openjdk.java.net/jdk/pull/2837