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>

compiler/rustc_lint/messages.ftl

@@ -536,9 +536,6 @@ lint_non_camel_case_type = {$sort} `{$name}` should have an upper camel case nam
     .suggestion = convert the identifier to upper camel case
     .label = should have an UpperCamelCase name
 
-lint_non_existent_doc_keyword = found non-existing keyword `{$keyword}` used in `#[doc(keyword = "...")]`
-    .help = only existing keywords are allowed in core/std
-
 lint_non_fmt_panic = panic message is not a string literal
     .note = this usage of `{$name}!()` is deprecated; it will be a hard error in Rust 2021
     .more_info_note = for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>