Remove useless alias from rustc_span to itself

Author: jyn514

src/librustdoc/lib.rs

@@ -53,7 +53,7 @@ extern crate rustc_parse;
 extern crate rustc_passes;
 extern crate rustc_resolve;
 extern crate rustc_session;
-extern crate rustc_span as rustc_span;
+extern crate rustc_span;
 extern crate rustc_target;
 extern crate rustc_trait_selection;
 extern crate rustc_typeck;