chore: remove eslint related contents

[minseolee]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [x] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [ ] It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• [ ] New/updated tests are included

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: It looks like ESLint has been replaced by Prettier. (72aed6a149b94b5b929fb47370a7a6d4cb7491c5) I've removed ESLint annotations, and updated some docs.

Minor things:

• I've ran npm run format (which executes prettier --check)
• Updated prettier version (2.6.2 -> 2.7.1)