Reuse RxJava built-in disposed Disposable (#1841)

diff --git a/reactive/kotlinx-coroutines-rx2/src/RxConvert.kt b/reactive/kotlinx-coroutines-rx2/src/RxConvert.kt
index 36e8dd9..0be606f 100644
--- a/reactive/kotlinx-coroutines-rx2/src/RxConvert.kt
+++ b/reactive/kotlinx-coroutines-rx2/src/RxConvert.kt
@@ -100,12 +100,7 @@
     }
 
     subscribe(observer)
-    awaitClose { disposableRef.getAndSet(Disposed)?.dispose() }
-}
-
-private object Disposed : Disposable {
-    override fun isDisposed() = true
-    override fun dispose() = Unit
+    awaitClose { disposableRef.getAndSet(Disposables.disposed())?.dispose() }
 }
 
 /**