[CHORE]: Remove `Plotly.Snapshot.toImage`

[camdecoster]

Description

Remove Plotly.Snapshot.toImage and its implementation in src/snapshot/toimage.js.

Why should this change be made?

• Plotly.toImage replaced it in 2016 (642ddba)
• The old function has no caller under src/ and no test coverage

Scope

• Delete src/snapshot/toimage.js
• Update/remove the existing references (src/snapshot/index.js:11, devtools/test_dashboard/devtools.js:86, src/snapshot/svgtoimg.js)

Notes

• This is a breaking change, so save it for the next major release
• This removes a public API through exports.Snapshot in src/core.js
• We could add a deprecation notice first