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.

library/core/src/convert/mod.rs

@@ -542,7 +542,7 @@ pub trait Into<T>: Sized {
 #[const_trait]
 pub trait From<T>: Sized {
     /// Converts to this type from the input type.
-    #[lang = "from"]
+    #[rustc_diagnostic_item = "from_fn"]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn from(value: T) -> Self;