RFR(T) 8224497: Remove FIXME in metaspaceClosure.cpp

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8224497

webrev: http://cr.openjdk.java.net/~ccheung/8224497/webrev.00/

A trivial fix to remove a leftover FIXME during the development of 
dynamic CDS archive.

I also take this opportunity to remove the comment and an unused 
function regarding the od (optional data) region.
The region was removed after the fix for JDK-8218751.

Testing: ran all AppCDS tests locally on linux-x64;
              running tiers1-3 testing using mach5.

thanks,
Calvin