RFR: 8298050: Add links to graph output for javadoc

[Per Minborg]

This PR proposes adding hyperlinks to the sealed graphic layout making navigation much simpler via the image.

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Commit messages:
 - Remove unused import
 - Add relative hyperlinks to nodes

Changes: https://git.openjdk.org/jdk/pull/11580/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11580&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298050
  Stats: 20 lines in 1 file changed: 19 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11580.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11580/head:pull/11580

PR: https://git.openjdk.org/jdk/pull/11580