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.

scripts/test_rustc_tests.sh

@@ -68,7 +68,6 @@ rm -r tests/run-make/split-debuginfo # same
 rm -r tests/run-make/symbols-include-type-name # --emit=asm not supported
 rm -r tests/run-make/target-specs # i686 not supported by Cranelift
 rm -r tests/run-make/mismatching-target-triples # same
-rm -r tests/run-make/use-extern-for-plugins # same
 rm tests/ui/asm/x86_64/issue-82869.rs # vector regs in inline asm not yet supported
 rm tests/ui/asm/x86_64/issue-96797.rs # const and sym inline asm operands don't work entirely correctly