[EME] PSA: behavior change - remove() must be explicitly called after load(); "released" key status

There was a change in behavior of persisted sessions that generate release
messages in
https://github.com/w3c/encrypted-media/commit/5c037a22fd08935f3ebe38cf2dc825ed91161eae.
I think this only affects retries where the keys were previously remove()'d
but the release message was not acknowledged.

Previously, load() would determine that the keys had been removed()'d and
re-send the release message. After the above change, which was part of pull
request #54, this message is not automatically sent. The application will
need to call remove() to re-generate this message.

Another change from that pull request is that there is a "released" key
status and keys that are removed are now "known" until their removal is
acknowledged. See
https://github.com/w3c/encrypted-media/commit/3acc89fbe44e35d9f1654a9c74da2abc40a752ba.
Among other things, this means that the keys are known in the load()
scenario above.

https://www.w3.org/wiki/HTML/Media_Task_Force/EME_Use_Cases#Persisted_License_.2F_Offline
needs to be updated to reflect this change in behavior.