remove `amp-viz-vega` experiment

[aghassemi]

Spec: #3824

• remove entry from tools/experiments.js
• remove isExperimentOn and related code in amp-viz-vega.js
• remove Experimental from the MD file amp-viz-vega.md
• remove Toggle Experiment button in examples/viz-vega.amp.html

[ericlindley-g]

@aghassemi — Slotting in "pending triage" for now. please update the milestone when you have a sense of when work will start. Thanks!

[erwinmombay]

@aghassemi want to confirm, this should still be in experimental correct?

[aghassemi]

yes

[adelinamart]

What's still pending here? Thanks

[adelinamart]

@aghassemi I think some of the these can be cleaned up. Thanks

[aghassemi]

Still experimental feature and not shipped.