Fix RunTimelineQuery double fetch on initial load (#21834)

## Summary & Motivation

Fix an initial double fetch

## How I Tested These Changes

Loaded a run timeline and saw only 1 RunTimelineQuery being made.


https://app.datadoghq.com/rum/sessions?query=%40type%3Aview%20%40application.id%3A6d12cca2-0263-463b-a90e-cc6c524e14bd%20%40view.name%3A%22%2Foverview%2Factivity%2Ftimeline%22&agg_m=count&agg_m_source=base&agg_q=%40issue.id&agg_q_source=base&agg_t=count&cols=&event=AgAAAY93sn8oo3AY8AAAAAAAAAAYAAAAAEFZOTNzbjhvQUFCaWZFUkVRa0xwbmtrSwAAACQAAAAAMDE4Zjc3YjItYjA0OC00ODBhLTgyNTQtZTVhNmZiNzc2YjM1&fromUser=true&graphType=flamegraph&shouldShowLegend=true&sort=time&spanID=6594261810473780884&tab=error&top_n=10&top_o=top&view=spans&viz=stream&x_missing=true&from_ts=1715699875871&to_ts=1715700175871&live=true
<img width="1215" alt="Screenshot 2024-05-14 at 11 23 24 AM"
src="https://github.com/dagster-io/dagster/assets/2286579/476d31fc-57ea-4466-b553-f3ca0cd01499">

js_modules/dagster-ui/packages/ui-core/src/overview/OverviewTimelineRoot.tsx

@@ -50,7 +50,6 @@ export const OverviewTimelineRoot = ({Header, TabButton}: Props) => {
   });
 
   React.useEffect(() => {
-    setNow(Date.now());
     const timer = setInterval(() => {
       setNow(Date.now());
     }, POLL_INTERVAL);