interop-testing: timeoutOnSleepingServer should fail on exception

This is ancient code, but our API should not throw IllegalStateException
in the case of races. This essentially reverts 0958fd4. The stream
code has changed a lot since this code was introduced, but at the very
least the replacement of AbstractStream that culminated in b661ac7
means the bug is almost certainly gone.

I ran Http2Test 1000 times and there were no failures.

Author: ejona86

interop-testing/src/main/java/io/grpc/testing/integration/AbstractInteropTest.java

@@ -1635,15 +1635,10 @@ public void timeoutOnSleepingServer() throws Exception {
     StreamObserver<StreamingOutputCallRequest> requestObserver
         = stub.fullDuplexCall(responseObserver);
 
-    StreamingOutputCallRequest request = StreamingOutputCallRequest.newBuilder()
+    requestObserver.onNext(StreamingOutputCallRequest.newBuilder()
         .setPayload(Payload.newBuilder()
             .setBody(ByteString.copyFrom(new byte[27182])))
-        .build();
-    try {
-      requestObserver.onNext(request);
-    } catch (IllegalStateException expected) {
-      // This can happen if the stream has already been terminated due to deadline exceeded.
-    }
+        .build());
 
     assertTrue(responseObserver.awaitCompletion(operationTimeoutMillis(), TimeUnit.MILLISECONDS));
     assertEquals(0, responseObserver.getValues().size());