Before we enabled mshots screenshots in /new/design, we generated the screenshots manually.

There's a feature flag that enables mshots, which can now be cleaned out. The script can also be removed.

Cleanup list:

• thumbnails generator script
• generator deps
• generator install script
• feature flag that enables mshots
• code that evaluates the feature flag above to render static designs in DesignPicker
• getDesignImageUrl util function in @automattic/design-picker package
• evaluate if should use DESIGN_IMAGE_FOLDER variable defined in @automattic/design-picker package
• the actual image files.