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_metadata/src/errors.rs

@@ -682,14 +682,6 @@ impl IntoDiagnostic<'_> for CannotFindCrate {
     }
 }
 
-#[derive(Diagnostic)]
-#[diag(metadata_no_dylib_plugin, code = "E0457")]
-pub struct NoDylibPlugin {
-    #[primary_span]
-    pub span: Span,
-    pub crate_name: Symbol,
-}
-
 #[derive(Diagnostic)]
 #[diag(metadata_crate_location_unknown_type)]
 pub struct CrateLocationUnknownType<'a> {