Source Bing Ads: remove the user_id parameter from spec.json

[sherifnada]

Current Behavior

The user_id parameter is not needed given that we can use the getuser endpoint to get it on behalf of the user which performed the oauth process

Expected Behavior

call getuser to get the user id instead of requiring it from the user

In fact, we can remove all the parameters required here: user_id, account_id, and campaign_id by following the steps provided here to get all those parameters for the given authenticated user.