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.

2 files changed, 0 insertions, 16 deletions

diff --git a/tests/ui/error-codes/E0463.rs b/tests/ui/error-codes/E0463.rs
deleted file mode 100644
index 683565281cf..00000000000
--- a/tests/ui/error-codes/E0463.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-#![feature(plugin)]
-#![plugin(cookie_monster)]
-//~^ ERROR E0463
-extern crate cake_is_a_lie;
-
-fn main() {
-}
diff --git a/tests/ui/error-codes/E0463.stderr b/tests/ui/error-codes/E0463.stderr
deleted file mode 100644
index 1aa66e1ec88..00000000000
--- a/tests/ui/error-codes/E0463.stderr
+++ /dev/null
@@ -1,9 +0,0 @@
-error[E0463]: can't find crate for `cookie_monster`
-  --> $DIR/E0463.rs:2:11
-   |
-LL | #![plugin(cookie_monster)]
-   |           ^^^^^^^^^^^^^^ can't find crate
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0463`.