RFR: 8202813: Move vm_weak processing from SystemDictionary::do_unloading to WeakProcessor

[Kim Barrett]

> On May 22, 2018, at 10:09 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> New webrev:
> http://cr.openjdk.java.net/~kbarrett/8202813/open.01/

I forgot to remove the no longer used is_alive argument for SystemDictionary::do_unloading.

Updated webrevs:
full: http://cr.openjdk.java.net/~kbarrett/8202813/open.02/
incr: http://cr.openjdk.java.net/~kbarrett/8202813/open.02.inc/