Remove the unmarked_api feature

Closes rust-lang#37981.

Author: jooert

reference.md

@@ -2463,11 +2463,6 @@ The currently implemented features of the reference compiler are:
 * `unboxed_closures` - Rust's new closure design, which is currently a work in
                        progress feature with many known bugs.
 
-* `unmarked_api` - Allows use of items within a `#![staged_api]` crate
-                   which have not been marked with a stability marker.
-                   Such items should not be allowed by the compiler to exist,
-                   so if you need this there probably is a compiler bug.
-
 * `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the
                               `#[allow_internal_unstable]` attribute, designed
                               to allow `std` macros to call