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_passes/src/errors.rs

@@ -717,10 +717,6 @@ pub enum MacroExport {
     TooManyItems,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(passes_plugin_registrar)]
-pub struct PluginRegistrar;
-
 #[derive(Subdiagnostic)]
 pub enum UnusedNote {
     #[note(passes_unused_empty_lints_note)]