Make register_[long_]diagnostics hygienic

src/libsyntax_ext/error_codes.rs

@@ -1,6 +1,6 @@
 #![allow(non_snake_case)]
 
-use syntax::{register_diagnostic, register_long_diagnostics};
+use syntax::register_long_diagnostics;
 
 // Error messages for EXXXX errors.
 // Each message should start and end with a new line, and be wrapped to 80 characters.