rustdoc: remove `type="text/css" from stylesheet links

MDN directly recommends this in <https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link>,
since "CSS is the only stylesheet language used on the web."

Like 07a243b2a4, but a few places that were
missed the first time.

src/librustdoc/html/static/js/main.js

@@ -184,7 +184,6 @@ function browserSupportsHistoryApi() {
 function loadCss(cssUrl) {
     const link = document.createElement("link");
     link.href = cssUrl;
-    link.type = "text/css";
     link.rel = "stylesheet";
     document.getElementsByTagName("head")[0].appendChild(link);
 }