Remove zone.js dependency

[alanorth]

Description

We do not use this directly and it seems to be a transitive dependency of Angular, so we should let Angular pull in the version it wants. You can see this causes dependency conflicts in the recent dependabot pull requests updating zone.js.

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• Remove zone.js dependency from package.json and yarn.lock

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[alanorth]

Ah, now I see we use it in src/main.browser.ts and src/main.server.ts for example. I was previously only looking in src/app.