Remove E0551.

Because it's the same as E0539.

Fixes #51489.

compiler/rustc_error_codes/src/error_codes/E0551.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler
+
 An invalid meta-item was used inside an attribute.
 
 Erroneous code example:
 
-```compile_fail,E0551
+```compile_fail,E0539
 #[deprecated(note)] // error!
 fn i_am_deprecated() {}
 ```