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_error_codes/src/error_codes/E0457.md

@@ -1,3 +1,5 @@
+#### Note: this error code is no longer emitted by the compiler`
+
 Plugin `..` only found in rlib format, but must be available in dylib format.
 
 Erroneous code example:

compiler/rustc_error_codes/src/error_codes/E0463.md

@@ -1,16 +1,13 @@
-A plugin/crate was declared but cannot be found.
+A crate was declared but cannot be found.
 
 Erroneous code example:
 
 ```compile_fail,E0463
-#![feature(plugin)]
-#![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
-extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`
+extern crate foo; // error: can't find crate
 ```
 
 You need to link your code to the relevant crate in order to be able to use it
-(through Cargo or the `-L` option of rustc example). Plugins are crates as
-well, and you link to them the same way.
+(through Cargo or the `-L` option of rustc, for example).
 
 ## Common causes
 

compiler/rustc_error_codes/src/error_codes/E0498.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The `plugin` attribute was malformed.
 
 Erroneous code example:
 
-```compile_fail,E0498
+```ignore (E0498 is no longer emitted)
 #![feature(plugin)]
 #![plugin(foo(args))] // error: invalid argument
 #![plugin(bar="test")] // error: invalid argument