Remove let_chains feature

compiler/rustc_parse/src/errors.rs

@@ -1778,6 +1778,13 @@ pub(crate) struct AsyncBoundModifierIn2015 {
     pub help: HelpUseLatestEdition,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_let_chain_pre_2024)]
+pub(crate) struct LetChainPre2024 {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_self_argument_pointer)]
 pub(crate) struct SelfArgumentPointer {