RFR(XS): 8204563: UseAppCDS obsolete message confusing

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8204563

webrev: http://cr.openjdk.java.net/~ccheung/8204563/webrev.00/

This change is for clarifying the obsolete message for the UseAppCDS option.

Existing:
     Ignoring option UseAppCDS; support was removed in 11.0

New:
     Ignoring obsolete option UseAppCDS; AppCDS is automatically enabled

Ran hs-tier1 and hs-tier2 testing successfully.

thanks,
Calvin