Remove mention of unsafe_no_drop_flag from Reference and Nomicon

src/doc/nomicon/safe-unsafe-meaning.md

@@ -26,10 +26,6 @@ can therefore be trusted. You can use `unsafe` on a trait implementation
 to declare that the implementation of that trait has adhered to whatever
 contracts the trait's documentation requires.
 
-There is also the `#[unsafe_no_drop_flag]` attribute, which exists for
-historic reasons and is being phased out. See the section on [drop flags]
-for details.
-
 The standard library has a number of unsafe functions, including:
 
 * `slice::get_unchecked`, which performs unchecked indexing, allowing

src/doc/reference.md

@@ -2059,10 +2059,6 @@ macro scope.
   outside of its dynamic extent), and thus this attribute has the word
   "unsafe" in its name. To use this, the
   `unsafe_destructor_blind_to_params` feature gate must be enabled.
-- `unsafe_no_drop_flag` - on structs, remove the flag that prevents
-  destructors from being run twice. Destructors might be run multiple times on
-  the same object with this attribute. To use this, the `unsafe_no_drop_flag` feature
-  gate must be enabled.
 - `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
 - `rustc_on_unimplemented` - Write a custom note to be shown along with the error
    when the trait is found to be unimplemented on a type.
@@ -2458,12 +2454,6 @@ 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.
 
-* `unsafe_no_drop_flag` - Allows use of the `#[unsafe_no_drop_flag]` attribute,
-                          which removes hidden flag added to a type that
-                          implements the `Drop` trait. The design for the
-                          `Drop` flag is subject to change, and this feature
-                          may be removed in the future.
-
 * `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,