The capacitors will self-discharge on their own if you remove the battery and power adaptor and wait a few hours before soaking the logic board. May I suggest you add a very little amount of dishwashing liquid to your soaking water to help free up the crud. After a few hours, use a soft toothbrush to gently scrub the gunk off. And pay attention to the cable connector sockets and the cable fittings that plug into them. Use pure distilled water to rinse thoroughly, then use a hair dryer to dry it up. Be warned there are areas not easily dried, like the space under the integrated circuit chips with legs on all sides. For these I use compressed air too. I use distilled drinking water, by the way. Available just about everywhere.
Please post your results so other readers can benefit too.