chore: drop `undici` (#56899)
### What?
Note: This is not a breaking change, just removing some unused code.
### Why?
Since #56896 we don't need this, as Node.js 18+ has `fetch` exposed by default.
### How?
Depends on #56896, #56909
We already didn't load `fetch` if `globalThis` had it (ie. Node.js 18+ environments), and since we are dropping support for Node.js 16, these code paths should have no effect on runtime behavior.
