Fix content warning being unnecessarily cleared when enabling/disabling CW (#13348)

app/javascript/mastodon/reducers/compose.js

@@ -261,7 +261,6 @@ export default function compose(state = initialState, action) {
     });
   case COMPOSE_SPOILERNESS_CHANGE:
     return state.withMutations(map => {
-      map.set('spoiler_text', '');
       map.set('spoiler', !state.get('spoiler'));
       map.set('idempotencyKey', uuid());