feat(replay): Cleanup viewed-by feature flag (frontend) (#69152)

Flag is here and released to 100% already:
https://flagr.getsentry.net/#/flags/601

Relates to https://github.com/getsentry/team-replay/issues/19
Relates to https://github.com/getsentry/sentry/issues/64924

static/app/components/events/eventReplay/replayPreviewPlayer.tsx

@@ -79,9 +79,6 @@ function ReplayPreviewPlayer({
 
   const {mutate: markAsViewed} = useMarkReplayViewed();
   useEffect(() => {
-    if (!organization.features.includes('session-replay-viewed-by-ui')) {
-      return;
-    }
     if (replayRecord && !replayRecord.has_viewed && !isFetching && isPlaying) {
       markAsViewed({projectSlug: replayRecord.project_id, replayId: replayRecord.id});
     }

static/app/views/replays/details.tsx

@@ -74,9 +74,6 @@ function ReplayDetails({params: {replaySlug}}: Props) {
 
   const {mutate: markAsViewed} = useMarkReplayViewed();
   useEffect(() => {
-    if (!organization.features.includes('session-replay-viewed-by-ui')) {
-      return;
-    }
     if (
       !fetchError &&
       replayRecord &&