Add note to E0550

With the change to built-in duplicate checking, E0550 is no longer
emitted.

compiler/rustc_error_codes/src/error_codes/E0550.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler
+
 More than one `deprecated` attribute has been put on an item.
 
 Erroneous code example:
 
-```compile_fail,E0550
+```compile_fail
 #[deprecated(note = "because why not?")]
 #[deprecated(note = "right?")] // error!
 fn the_banished() {}