Remove identity_future from stdlib

This function/lang_item was introduced in rust-lang#104321 as a temporary workaround of future lowering.
The usage and need for it went away in rust-lang#104833.
After a bootstrap update, the function itself can be removed from `std`.

crates/hir-def/src/lang_item.rs

@@ -379,7 +379,6 @@ language_item_table! {
  // FIXME(swatinem): the following lang items are used for async lowering and
  // should become obsolete eventually.
  ResumeTy, ResumeTy, resume_ty, Target::Struct, GenericRequirement::None;
- IdentityFuture, identity_future, identity_future_fn, Target::Fn, GenericRequirement::None;
  GetContext, get_context, get_context_fn, Target::Fn, GenericRequirement::None;
 
  Context, Context, context, Target::Struct, GenericRequirement::None;