Auto merge of rust-lang#116412 - nnethercote:rm-plugin-support, r=bjorn3

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 rust-lang#29597.

r? `@ghost`

Author: bors

core/src/ops/mod.rs

@@ -8,8 +8,8 @@
 //! trait, but since the assignment operator (`=`) has no backing trait, there
 //! is no way of overloading its semantics. Additionally, this module does not
 //! provide any mechanism to create new operators. If traitless overloading or
-//! custom operators are required, you should look toward macros or compiler
-//! plugins to extend Rust's syntax.
+//! custom operators are required, you should look toward macros to extend
+//! Rust's syntax.
 //!
 //! Implementations of operator traits should be unsurprising in their
 //! respective contexts, keeping in mind their usual meanings and