Remove unstable-book documentation

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-# `tool_lints`
-
-The tracking issue for this feature is: [#44690]
-
-[#44690]: https://github.com/rust-lang/rust/issues/44690
-
-------------------------
-
-Tool lints let you use scoped lints, to `allow`, `warn`, `deny` or `forbid` lints of
-certain tools.
-
-Currently `clippy` is the only available lint tool.
-
-It is recommended for lint tools to implement the scoped lints like this:
-
-- `#[_(TOOL_NAME::lintname)]`: for lint names
-- `#[_(TOOL_NAME::lintgroup)]`: for groups of lints
-- `#[_(TOOL_NAME::all)]`: for (almost[^1]) all lints
-
-## An example
-
-```rust
-#![feature(tool_lints)]
-
-#![warn(clippy::pedantic)]
-
-#[allow(clippy::filter_map)]
-fn main() {
-    let v = vec![0; 10];
-    let _ = v.into_iter().filter(|&x| x < 1).map(|x| x + 1).collect::<Vec<_>>();
-    println!("No filter_map()!");
-}
-```
-
-[^1]: Some defined lint groups can be excluded here.