Remove `rustc::existing_doc_keyword` lint.

`CheckAttrVisitor::check_doc_keyword` checks `#[doc(keyword = "..")]`
attributes to ensure they are on an empty module, and that the value is
a non-empty identifier.

The `rustc::existing_doc_keyword` lint checks these attributes to ensure
that the value is the name of a keyword.

It's silly to have two different checking mechanisms for these
attributes. This commit does the following.
- Changes `check_doc_keyword` to check that the value is the name of a
  keyword (avoiding the need for the identifier check, which removes a
  dependency on `rustc_lexer`).
- Removes the lint.
- Updates tests accordingly.

There is one hack: the `SelfTy` FIXME case used to used to be handled by
disabling the lint, but now is handled with a special case in
`is_doc_keyword`. That hack will go away if/when the FIXME is fixed.

Co-Authored-By: Guillaume Gomez <guillaume1.gomez@gmail.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/rustdoc-gui/src/test_docs/lib.rs b/tests/rustdoc-gui/src/test_docs/lib.rs
index 91aa2c3fae5..dae13d9ac18 100644
--- a/tests/rustdoc-gui/src/test_docs/lib.rs
+++ b/tests/rustdoc-gui/src/test_docs/lib.rs
@@ -156,7 +156,7 @@ pub enum AnEnum {
     WithVariants { and: usize, sub: usize, variants: usize },
 }
 
-#[doc(keyword = "CookieMonster")]
+#[doc(keyword = "for")]
 /// Some keyword.
 pub mod keyword {}