RFR: Remove unused visualizer option

[William Kemper]

This option should have been removed when we switched the region sampling to use unified logging framework.

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Commit messages:
 - Remove unused visualizer option

Changes: https://git.openjdk.org/shenandoah/pull/217/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=217&range=00
  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/217.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/217/head:pull/217

PR: https://git.openjdk.org/shenandoah/pull/217