[core] throw AssertionError if SessionId is missing

Also remove stale comment

briar-core/src/main/java/org/briarproject/briar/introduction/IntroductionManagerImpl.java

@@ -583,9 +583,11 @@ class IntroductionManagerImpl extends ConversationClientImpl
 				throw new DbException(e);
 			}
 			if (m.getSessionId() == null) {
-				// this can only be an unhandled REQUEST message
-				// that does not yet have a SessionId assigned
-				continue;
+				// This can only be an unhandled REQUEST message.
+				// Its session is created and stored in incomingMessage(),
+				// and getMessageMetadata() only returns delivered messages,
+				// so the session ID should have been assigned.
+				throw new AssertionError("missing session ID");
 			}
 			// get session from map or database
 			DeletableSession session = sessions.get(m.getSessionId());

briar-core/src/test/java/org/briarproject/briar/introduction/IntroductionIntegrationTest.java

@@ -1349,11 +1349,6 @@ public class IntroductionIntegrationTest
 		assertFalse(deleteAllMessages0From2());
 	}
 
-	/**
-	 * This test is testing that a session's deletable flag gets reset
-	 * when the session is used again,
-	 * so that it can not cause a session to get deleted prematurely.
-	 */
 	@Test
 	public void testDeletingOneSideOfSession() throws Exception {
 		addListeners(false, false);