Removed built-in extensions

[andrewpareles]

On commit 91f83212788ced6e6bc65adf75acaeb66ef8cb14 and prior, the product.json file provides some built-in extensions. However, when you try bundling Void, the gulpfile script detects these are outdated, tries updating them, crashes (because we're using open-vsx), and fails.

To reproduce this without waiting for the whole build, you can run npm run gulp compile-extensions-build-pr. Also see npm run download-builtin-extensions.

I deleted these built-ins for now so we can bundle, but we might want to re-add them in the future.