Replace diagnostic plugins with macro_rules

src/libsyntax_ext/error_codes.rs

@@ -1,5 +1,3 @@
-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.  In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use