Remove unused feature from error index generator

Remove feature `rustdoc`.

src/tools/error_index_generator/main.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(rustc_private, rustdoc)]
+#![feature(rustc_private)]
 
 extern crate syntax;
 extern crate rustdoc;