Remove section on stability levels from reference

Obsolete, no replacement. The mechanics here are mostly implementation details at this point.

Author: brson

src/doc/reference.md

@@ -2493,77 +2493,6 @@ Supported traits for `derive` are:
 * `Show`, to format a value using the `{}` formatter.
 * `Zero`, to create a zero instance of a numeric data type.
 
-### Stability
-
-One can indicate the stability of an API using the following attributes:
-
-* `deprecated`: This item should no longer be used, e.g. it has been
-  replaced. No guarantee of backwards-compatibility.
-* `experimental`: This item was only recently introduced or is
-  otherwise in a state of flux. It may change significantly, or even
-  be removed. No guarantee of backwards-compatibility.
-* `unstable`: This item is still under development, but requires more
-  testing to be considered stable. No guarantee of backwards-compatibility.
-* `stable`: This item is considered stable, and will not change
-  significantly. Guarantee of backwards-compatibility.
-* `frozen`: This item is very stable, and is unlikely to
-  change. Guarantee of backwards-compatibility.
-* `locked`: This item will never change unless a serious bug is
-  found. Guarantee of backwards-compatibility.
-
-These levels are directly inspired by
-[Node.js' "stability index"](http://nodejs.org/api/documentation.html).
-
-Stability levels are inherited, so an item's stability attribute is the default
-stability for everything nested underneath it.
-
-There are lints for disallowing items marked with certain levels: `deprecated`,
-`experimental` and `unstable`. For now, only `deprecated` warns by default, but
-this will change once the standard library has been stabilized. Stability
-levels are meant to be promises at the crate level, so these lints only apply
-when referencing items from an _external_ crate, not to items defined within
-the current crate. Items with no stability level are considered to be unstable
-for the purposes of the lint. One can give an optional string that will be
-displayed when the lint flags the use of an item.
-
-For example, if we define one crate called `stability_levels`:
-
-```{.ignore}
-#[deprecated="replaced by `best`"]
-pub fn bad() {
-    // delete everything
-}
-
-pub fn better() {
-    // delete fewer things
-}
-
-#[stable]
-pub fn best() {
-    // delete nothing
-}
-```
-
-then the lints will work as follows for a client crate:
-
-```{.ignore}
-#![warn(unstable)]
-extern crate stability_levels;
-use stability_levels::{bad, better, best};
-
-fn main() {
-    bad(); // "warning: use of deprecated item: replaced by `best`"
-
-    better(); // "warning: use of unmarked item"
-
-    best(); // no warning
-}
-```
-
-> **Note:** Currently these are only checked when applied to individual
-> functions, structs, methods and enum variants, *not* to entire modules,
-> traits, impls or enums themselves.
-
 ### Compiler Features
 
 Certain aspects of Rust may be implemented in the compiler, but they're not