Remove E0551.

Because it's the same as E0539.

Fixes #51489.

compiler/rustc_attr/src/builtin.rs

@@ -846,7 +846,7 @@ pub fn find_deprecation(
                                 ),
                             );
                         } else {
-                            sess.emit_err(session_diagnostics::IncorrectMetaItem2 {
+                            sess.emit_err(session_diagnostics::IncorrectMetaItem {
                                 span: meta.span,
                             });
                         }

compiler/rustc_attr/src/session_diagnostics.rs

@@ -165,15 +165,6 @@ pub(crate) struct MissingIssue {
     pub span: Span,
 }
 
-// FIXME: This diagnostic is identical to `IncorrectMetaItem`, barring the error code. Consider
-// changing this to `IncorrectMetaItem`. See #51489.
-#[derive(Diagnostic)]
-#[diag(attr_incorrect_meta_item, code = "E0551")]
-pub(crate) struct IncorrectMetaItem2 {
-    #[primary_span]
-    pub span: Span,
-}
-
 // FIXME: Why is this the same error code as `InvalidReprHintNoParen` and `InvalidReprHintNoValue`?
 // It is more similar to `IncorrectReprFormatGeneric`.
 #[derive(Diagnostic)]

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() {}
 ```