The rule for nested lists in `normalize.css` and `opinionated.css` is no longer necessary

The rule "Remove the margin on nested lists in Chrome, Edge, and Safari" appears in both normalize.css and opinionated.css:

https://github.com/csstools/normalize.css/blob/0774d1043e07ba868f157ff7bb1cd0bbb0a2871c/normalize.css#L32-L39

https://github.com/csstools/normalize.css/blob/0774d1043e07ba868f157ff7bb1cd0bbb0a2871c/opinionated.css#L40-L47

Since Chromium and WebKit already have that rule in their UA stylesheets, I think it can be removed.

- Chromium [fixed it 1 year ago](https://chromium-review.googlesource.com/c/chromium/src/+/6629436): https://github.com/chromium/chromium/blob/0fa7607bb96dc68c2d08d9920ce5da6bc4d3287a/third_party/blink/renderer/core/html/resources/html.css#L358-L365
- WebKit [fixed it 3 years ago](https://github.com/WebKit/WebKit/commit/5febb62e57ba1916cc54cc3cfafc7ac5aef9f003): https://github.com/WebKit/WebKit/blob/b524a4ebf3a70a7b6409c55db9eae74800721dc7/Source/WebCore/css/html.css#L308-L315

Start by comparing the nested-list rules in normalize.css and opinionated.css at the linked locations, then review the referenced Chromium and WebKit user-agent stylesheet changes. Remove the no-longer-needed rules from both files and confirm the project’s CSS checks or tests still pass.

索引モデルが issue の本文から書いたものです。