rustdoc: remove no-op CSS .impl { flex-basis: 100% } etc

When `.impl-items { flex-basis: 100% }` and `h3.impl, h3.method, h4.method,
h3.type, h4.type, h4.associatedconstant` were added in
34bd2b845b3acd84c5a9bddae3ff8081c19ec5e9, it seems like it was a mistake even
then. According to MDN, [flex-basis] does nothing unless the box it's applied
to is a flex *item*, a child of a flex container. However, when this was
added, these elements were flex containers themselves.

[flex-basis]: https://developer.mozilla.org/en-US/docs/Web/CSS/flex-basis

src/librustdoc/html/static/css/rustdoc.css

@@ -193,7 +193,6 @@ h4.code-header {
 .methods .associatedconstant,
 .impl-items .associatedtype,
 .methods .associatedtype {
-	flex-basis: 100%;
 	position: relative;
 }