docs(query-cancellation): remove more references to old cancel fn (#4631)

Apparently these were missed in https://github.com/TanStack/query/commit/a1050eb0d7c7c44731fc7e6839e77f69f83bab67.

It is confusing because the [Migration to v4 guide](https://tanstack.com/query/v4/docs/guides/migrating-to-react-query-4#the-cancel-method-on-promises-is-no-longer-supported) 
states that the `cancel` property in promises is no longer supported, but the docs for Query Cancellation seem to imply that it is.

I assume that it is not supported anymore based on the commit linked above, the migration guide and my own testing.
If that is indeed the case, let's clean this up.