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 #29597.

compiler/rustc_feature/src/removed.rs

@@ -152,9 +152,12 @@ declare_features! (
      Some("removed in favor of `#![feature(marker_trait_attr)]`")),
     (removed, panic_implementation, "1.28.0", Some(44489), None,
      Some("subsumed by `#[panic_handler]`")),
+    /// Allows using `#![plugin(myplugin)]`.
+    (removed, plugin, "CURRENT_RUSTC_VERSION", Some(29597), None,
+     Some("plugins are no longer supported")),
     /// Allows using `#[plugin_registrar]` on functions.
     (removed, plugin_registrar, "1.54.0", Some(29597), None,
-     Some("a __rustc_plugin_registrar symbol must now be defined instead")),
+     Some("plugins are no longer supported")),
     (removed, proc_macro_expr, "1.27.0", Some(54727), None,
      Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
     (removed, proc_macro_gen, "1.27.0", Some(54727), None,