Source Bing Ads: redirect_url is not needed

[sherifnada]

Current Behavior

The redirect_url parameter requested by the connector is not needed. You'll notice that the connector only uses the method request_oauth_tokens_by_refresh_tokens which does not require the use of redirect_url. Therefore it's not required for the parameter to be included in spec.json and should be removed.

Expected Behavior

remove the parameter from spec.json in a backwards compatible way (keeping additionalProperties=true so as not to break the config for existing users)