RFR: 8202083: Remove explicit CMS checks in CardTableBarrierSet

[Erik Österlund]

Hi,

In CardTableBarrierSet, we sometimes need fencing due to the card table 
being scanned concurrently when using CMS with pre-cleaning. Rather than 
explicitly checking for those CMS flags in the generic 
CardTableBarrierSet class, it is preferrable to check the CardTable 
scanned_concurrently() property which reflects that better

Webrev:
http://cr.openjdk.java.net/~eosterlund/8202083/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8202083

Thanks,
/Erik