Migrate BuiltinLintDiag::MissingAbi to use LintDiagnostic directly

compiler/rustc_lint/src/lints.rs

@@ -1,7 +1,6 @@
 #![allow(rustc::untranslatable_diagnostic)]
 use std::num::NonZero;
 
-use rustc_abi::ExternAbi;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagArgValue, DiagMessage, DiagStyledString, ElidedLifetimeInPathSubdiag,
@@ -2879,14 +2878,6 @@ pub(crate) struct PatternsInFnsWithoutBodySub {
     pub ident: Ident,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_extern_without_abi)]
-pub(crate) struct MissingAbi {
-    #[suggestion(code = "extern {default_abi}", applicability = "machine-applicable")]
-    pub span: Span,
-    pub default_abi: ExternAbi,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(lint_legacy_derive_helpers)]
 pub(crate) struct LegacyDeriveHelpers {