remove deprecated .cancel on promises

[TkDodo]

With the addition of out-of-the-box support for Query Cancellation via AbortController and AbortSignal (v3.30.0) we will remove the old way to cancel Promises by providing a .cancel method.

• remove old code
• update docs

[TkDodo]

@jcready let me know if you're interested in working on this, given that you're the mastermind behind the query cancellation :)

[prateek3255]

@TkDodo I would like to give this a shot