[whatwg/fetch] "no-CORS-safelisted request-header" definition can be simplified (#898)

Step 1 of the definition of [no-CORS-safelisted request-header](https://fetch.spec.whatwg.org/#no-cors-safelisted-request-header) can be dropped completely, it's implicitly checked by step 2 of [CORS-safelisted request-header](https://fetch.spec.whatwg.org/#cors-safelisted-request-header).

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