Don't add rustdoc's CSS to other doc pages

This was originally added so those doc pages could use the same font
files, but it turns out to be fragile. And those doc pages are just
stubs that link to other pages, so they don't need fancy fonts.

src/bootstrap/doc.rs

@@ -383,15 +383,9 @@ impl Step for Standalone {
             }
 
             if filename == "not_found.md" {
-                cmd.arg("--markdown-css")
-                    .arg(format!("https://doc.rust-lang.org/rustdoc{}.css", &builder.version))
-                    .arg("--markdown-css")
-                    .arg("https://doc.rust-lang.org/rust.css");
+                cmd.arg("--markdown-css").arg("https://doc.rust-lang.org/rust.css");
             } else {
-                cmd.arg("--markdown-css")
-                    .arg(format!("rustdoc{}.css", &builder.version))
-                    .arg("--markdown-css")
-                    .arg("rust.css");
+                cmd.arg("--markdown-css").arg("rust.css");
             }
             builder.run(&mut cmd);
         }