Remove *src attributes

[emilykl]

Addresses #7623

Schema attributes ending in src existed only to work with Chart Studio features, so they are no longer necessary

This PR:

• Removes all *src attributes and associated logic
• Removes layout.hideSources attribute which was also only needed for Chart Studio
• Updates tests accordingly (including deleting these removed attributes from image mocks)
• Updates typegen script
• Regenerates plot-schema.json and generated types

[emilykl]

This second draftlog is for PR #7812, which we forgot to add a draftlog for at the time.