rustc: Do not link to plugins

This flag seems to have erroneously been set to `true`.

src/librustc/metadata/creader.rs

@@ -462,7 +462,7 @@ pub fn read_plugin_metadata<'b>(&'b mut self,
                      name: s.to_string(),
                      ident: s.to_string(),
                      id: ast::DUMMY_NODE_ID,
-                     should_link: true,
+                     should_link: false,
                  }, sp)
             }
         };