Rollup merge of #119425 - Urgau:check-cfg-fix-cargo-diag-bug, r=Nilstrieb

Fix invalid check-cfg Cargo feature diagnostic help

#118213 added specialized diagnostic for Cargo `feature` cfg. However when providing an empty `#[cfg(feature)]` condition the suggestion would suggest adding `feature` as a feature in `Cargo.toml` (wtf!).

This PR removes the invalid logic, which even brings a nice improvement.

```diff
   --> $DIR/cargo-feature.rs:18:7
    |
 LL | #[cfg(feature)]
-   |       ^^^^^^^
+   |       ^^^^^^^- help: specify a config value: `= "bitcode"`
    |
    = note: expected values for `feature` are: `bitcode`
-   = help: consider defining `feature` as feature in `Cargo.toml`
```

The first commit add a test showing the bug and the second commit fixes the bug.

`@rustbot` label +F-check-cfg

compiler/rustc_lint/src/context.rs

@@ -804,8 +804,6 @@ pub trait LintContext {
                                 db.span_suggestion(value_span, "there is a expected value with a similar name", format!("\"{best_match}\""), Applicability::MaybeIncorrect);
 
                             }
-                        } else if name == sym::feature && is_from_cargo {
-                            db.help(format!("consider defining `{name}` as feature in `Cargo.toml`"));
                         } else if let &[first_possibility] = &possibilities[..] {
                             db.span_suggestion(name_span.shrink_to_hi(), "specify a config value", format!(" = \"{first_possibility}\""), Applicability::MaybeIncorrect);
                         }