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.

1 files changed, 0 insertions, 4 deletions

diff --git a/compiler/rustc_plugin_impl/messages.ftl b/compiler/rustc_plugin_impl/messages.ftl
deleted file mode 100644
index 8db32a42c1d..00000000000
--- a/compiler/rustc_plugin_impl/messages.ftl
+++ /dev/null
@@ -1,4 +0,0 @@
-plugin_impl_load_plugin_error = {$msg}
-
-plugin_impl_malformed_plugin_attribute = malformed `plugin` attribute
-    .label = malformed attribute