JVM_LatestUserDefinedLoader can be removed. The stack walking can be done in java.

We should have java.io.ObjectInputStream and java.rmi.MarshalInputStream which are the two clients of jdk.internal.misc.VM.latestUserDefinedLoader to do the stack walking itself and remove jdk.internal.misc.VM.latestUserDefinedLoader entirely. It's not a good practice to define an API to walk the stack to find the first user-defined loader.