Replace "dead code" with "unused code"

[kaycebasques]

This is a request from @connorjclark: https://github.com/GoogleChrome/web.dev/pull/3266/files#r441937465

I'm not sure his rationale for replacing it but I can guess: "unused" is more technically accurate than "dead" and "dead" might bring up unwanted emotions for some people.

[jakearchibald]

This is a tough one. tooling.report is documenting the bundlers, and the bundlers all refer to this feature as removing "dead code". If we invent our own term for it, does that make the documentation harder to understand?

• https://rollupjs.org/guide/en/#:~:text=dead%20code
• https://webpack.js.org/guides/tree-shaking/#:~:text=dead-code
• https://parceljs.org/production.html#:~:text=dead_code

[developit]

I see the point of moving to a less charged term for this, but my concern is the same as Jake's - "Dead Code Elimination" is the established name for this, and changing that really would need to start with more than just tooling.report picking another name.

One option we could consider: refer to it as "unused code" when discussing the concept and details, but still acknowledge that the technique is most commonly known as "Dead Code Elimination".

[jakearchibald]

We could just go with "Tree shaking", even though that's a subset