Remove flaky assertion in EME polyfill.

This assertion did not add much value, and it sometimes triggers a
test failure.  Since prefixed EME does not have actual session
management, it's entirely possible for a stray message to come in
for a session we've already destroyed in the polyfill.  Rather than
assert, just ignore it.

Change-Id: I4779d3f36a25bc21ade95c4ea3f25ad1f8a96e7b

lib/polyfill/patchedmediakeys_v01b.js

@@ -390,7 +390,6 @@ shaka.polyfill.PatchedMediaKeys.v01b.MediaKeys.prototype.onWebkitKeyMessage_ =
   shaka.log.debug('v01b.onWebkitKeyMessage_', event);
 
   var session = this.findSession_(event.sessionId);
-  shaka.asserts.assert(session);
   if (!session) {
     shaka.log.error('Session not found', event.sessionId);
     return;