RFR: 8210065: ZGC: Remove mode for treating weaks as strong

[Erik Österlund]

Hi,

In ZGC, the root processor has a mode for supporting treating weaks as 
strong. This makes no sense to use for a user as it only introduces 
memory leaks. We have supported it anyway because it was a neat 
debugging tool while implementing concurrent reference processing. But 
now it seems like it is time for this option to retire, as it makes the 
root scanning code unnecessarily complicated and bug prone.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8210065/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8210065

Thanks,
/Erik