RFR(S): 8213000: Obsolete the IgnoreUnverifiableClassesDuringDump vm option

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8213000

webrev: http://cr.openjdk.java.net/~ccheung/8213000/webrev.00/

Please review this change for obsoleting the diagnostic 
IgoreUnverifiableClassesDuringDump vm option in JDK12.
With the change, if the option is specified in the command line, the 
user will see the following message:

Java HotSpot(TM) 64-Bit Server VM warning: Ignoring option 
IgnoreUnverifiableClassesDuringDump; support was removed in 12.0

Testing:
     Ran all CDS and AppCDS tests locally on linux-x64.
     hs-tier{1,2,3} testing in progress.

thanks,
Calvin