build: don't store eslint locally

This PR removes eslint as a local dependency and switches to downloading it from npm. It will still be updated regularly via @dependabot.

Why?

1. eslint is the only linter stored locally (besides cpplint, which has patches, unlike ESLint).
2. There are multiple issues related to file paths being too long and tarballs failing to lint properly (see: #39709 (closed), #34005 (closed), #32765 (closed), and others).

Why was eslint set up like this initially?

When eslint was first added to the repository in #1539 around 10 years, it replaced a (not as large, but still large) library, closure-lint. Since then, the library has almost 4x its size. At the time, there was no discussion (AFAICT) about keeping or removing the directory—it was simply included as it was.

Size-wise, this'll save ~40MB, which isn't a huge much, but it's a decent amount.