interop-testing: Do not System.exit(0) from interop client

After #9937 was merged, the Java observability tests start to fail.

This System.exit(0) call in the existing Interop client main() method
prevented execution to continue in the new combined Observability
Interop test binary here. (The new binary is calling the old binary's
main() method.)

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

@@ -71,7 +71,6 @@ public static void main(String[] args) throws Exception {
     } finally {
       client.tearDown();
     }
-    System.exit(0);
   }
 
   private String serverHost = "localhost";