Fix participants disappearing during focus switch (probably)

I discovered that this hook was calling complete on the returned observable almost immediately when it gets mounted. This caused the call view model to never know when the application was switching focuses. At first I thought this was just because I forgot to move the call to complete to the effect's clean-up function, but even with that changed, React still calls the effect twice in strict mode. So, let's just remove the call entirely.

src/state/useObservable.ts

@@ -1,5 +1,5 @@
 /*
-Copyright 2023 New Vector Ltd
+Copyright 2023-2024 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,18 +14,17 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { useEffect, useRef } from "react";
+import { useRef } from "react";
 import { BehaviorSubject, Observable } from "rxjs";
 
 /**
  * React hook that creates an Observable from a changing value. The Observable
- * replays its current value upon subscription, emits whenever the value
- * changes, and completes when the component is unmounted.
+ * replays its current value upon subscription and emits whenever the value
+ * changes.
  */
 export function useObservable<T>(value: T): Observable<T> {
   const subject = useRef<BehaviorSubject<T>>();
   subject.current ??= new BehaviorSubject(value);
   if (value !== subject.current.value) subject.current.next(value);
-  useEffect(() => subject.current!.complete(), []);
   return subject.current;
 }