RFR: 8202082: Remove explicit CMS checks in CardTableBarrierSetAssembler

[Erik Österlund]

Hi,

In CardTableBarrierSetAssembler, 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 
CardTableBarrierSetAssembler class, it is preferrable to check the 
CardTable scanned_concurrently() property which already precisely 
reflects that.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8202082

Webrev:
http://cr.openjdk.java.net/~eosterlund/8202082/webrev.00/

Thanks,
/Erik