RFR[T]: 8240133: G1DirtyCardQueue destructor has useless flush

[Kim Barrett]

Please review this trivial change to remove the useless call to flush() from
the G1DirtyCardQueue destructor.  See the CR for more details.  This removes
the need for a non-trivial destructor for that class.

CR:
https://bugs.openjdk.java.net/browse/JDK-8240133

Webrev:
https://cr.openjdk.java.net/~kbarrett/8240133/open.00/

Testing:
mach5 tier1-5 along with changes for JDK-8239825 and JDK-8139652.  
Local (linux-x64) hotspot:tier1 with this and the proposed JDK-8239825 change.