internal: remove redundant disable 'unused_variables'

[A4-Tacks]

Fixup #23043

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BTW, #23043 has another advantage, it won't be caught by unused_variables's fix when applying line quick fix

fn main() {
    $0let Some(x) = Some(2);
}

Before:

fn main() {
    let Some(_x) = Some(2);
}

After (better):

fn main() {
    let Some(x) = Some(2) else { return };
}