Remove support for compiler plugins.

They've been deprecated for four years.

This commit includes the following changes.
- It eliminates the `rustc_plugin_impl` crate.
- It changes the language used for lints in
  `compiler/rustc_driver_impl/src/lib.rs` and
  `compiler/rustc_lint/src/context.rs`. External lints are now called
  "loaded" lints, rather than "plugins" to avoid confusion with the old
  plugins. This only has a tiny effect on the output of `-W help`.
- E0457 and E0498 are no longer used.
- E0463 is narrowed, now only relating to unfound crates, not plugins.
- The `plugin` feature was moved from "active" to "removed".
- It removes the entire plugins chapter from the unstable book.
- It removes quite a few tests, mostly all of those in
  `tests/ui-fulldeps/plugin/`.

Closes rust-lang#29597.

Author: nnethercote

crates/hir-def/src/attr/builtin.rs

@@ -262,24 +262,6 @@ pub const INERT_ATTRIBUTES: &[BuiltinAttribute] = &[
         naked_functions, experimental!(naked)
     ),
 
-    // Plugins:
-    // BuiltinAttribute {
-    //     name: sym::plugin,
-    //     only_local: false,
-    //     type_: CrateLevel,
-    //     template: template!(List: "name"),
-    //     duplicates: DuplicatesOk,
-    //     gate: Gated(
-    //         Stability::Deprecated(
-    //             "https://github.com/rust-lang/rust/pull/64675",
-    //             Some("may be removed in a future compiler version"),
-    //         ),
-    //         sym::plugin,
-    //         "compiler plugins are deprecated",
-    //         cfg_fn!(plugin)
-    //     ),
-    // },
-
     // Testing:
     gated!(
         test_runner, CrateLevel, template!(List: "path"), ErrorFollowing, custom_test_frameworks,