Deprecate AllowJNIEnvProxy

The product flag, AllowJNIEnvProxy, was added in JDK 6 by JDK-4881230 as the first step in the process of removing support for JNIEnv proxies (which the JNI spec forbids). It's unclear how long ago the need for JNIEnv proxies was removed, and more unclear when they would actually have stopped working. I have great confidence that the code has not worked for a very long time.

So it is time to complete that process and remove the last remnants of JNIEnv proxy support. As AllowJNIEnvProxy is a product flag we will do the right think and initially deprecate it in JDK 13 and then obsolete in 14, then remove.