Remove redundant SNEWS handler key
This is the same issue fixed for GRBS in !514 (merged) . This issues causes the RAVEN pipeline to be triggered twice when listening to SNEWS events (see example of this: [https://gracedb-playground.ligo.org/superevents/S191016gu/view/])(https://gracedb-playground.ligo.org/superevents/S191016gu/view/), see resulting sentry error: https://sentry.io/organizations/ligo-caltech/issues/1323983403/?query=is%3Aunresolved).