chore: remove credentials typings from fetch

[crowlKats]

Deno doesn't handle the credentials option, so the inclusion of typings is miss-informing. Also in the case of Request's readonly credentials property, which isnt set at all, deno's behaviour conflicts with the typings

[MierenManz]

wouldn't this technically be a breaking change?

[lucacasonato]

This makes Deno less compatible with ts code targeting browsers and Deno so we won't land it. Thanks though 🙂

[crowlKats]

one problem though is we dont set the reaedonly credentials property, so its undefined when accessed, which doesnt match typings at all