Remove isAsyncStarted assertion

Issue: SPR-14444

spring-web/src/main/java/org/springframework/web/context/request/async/WebAsyncManager.java

@@ -105,7 +105,6 @@ public final class WebAsyncManager {
 	 */
 	public void setAsyncWebRequest(final AsyncWebRequest asyncWebRequest) {
 		Assert.notNull(asyncWebRequest, "AsyncWebRequest must not be null");
-		Assert.state(!isConcurrentHandlingStarted(), "Can't set AsyncWebRequest with concurrent handling in progress");
 		this.asyncWebRequest = asyncWebRequest;
 		this.asyncWebRequest.addCompletionHandler(new Runnable() {
 			@Override