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_lint/src/lib.rs

@@ -10,7 +10,7 @@
 //! all other analyses. The `LintPass`es built into rustc are defined
 //! within [rustc_session::lint::builtin],
 //! which has further comments on how to add such a lint.
-//! rustc can also load user-defined lint plugins via the plugin mechanism.
+//! rustc can also load external lint plugins, as is done for Clippy.
 //!
 //! Some of rustc's lints are defined elsewhere in the compiler and work by
 //! calling `add_lint()` on the overall `Session` object. This works when