RFR 15 (S) 8231559: Remove expired flags in JDK 15

[David Holmes]

It is that time of the release again. All the expired entries in the 
flag table can be deleted.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231559
webrev: http://cr.openjdk.java.net/~dholmes/8231559/webrev/

It's mostly all the old CMS flags.

Thanks,
David