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/messages.ftl

@@ -196,9 +196,6 @@ metadata_newer_crate_version =
 metadata_no_crate_with_triple =
     couldn't find crate `{$crate_name}` with expected target triple {$locator_triple}{$add_info}
 
-metadata_no_dylib_plugin =
-    plugin `{$crate_name}` only found in rlib format, but must be available in dylib format
-
 metadata_no_link_mod_override =
     overriding linking modifiers from command line is not supported