'Open to the Side' button in context menu for markdown preview should be removed

[sbatten]

Issue Type: Bug

1. Open a markdown preview.
2. Right-click the preview in explorer
3. Select open to the side
4. Nothing happens

This button should be removed as per new model.

VS Code version: Code - Insiders 1.21.0-insider (1a84a88, 2018-02-27T09:01:48.555Z)
OS version: Windows_NT x64 10.0.16299

Reproduces without extensions