Remove from_fn lang item

It was probably a leftover from the old `?` desugaring but anyways, it's
unused now except for clippy, which can just use a diagnostics item.

compiler/rustc_span/src/symbol.rs

@@ -740,6 +740,7 @@ symbols! {
         frem_fast,
         from,
         from_desugaring,
+        from_fn,
         from_iter,
         from_method,
         from_output,