Remove duplicated unregistered classes from dynamic archive

In the CDS archive, "unregistered" classes are those to be loaded by custom loaders.

When doing a CDS dynamic dump (with -XX:ArchiveClassesAtExit=foo.jsa), a class of the same name may be loaded by several custom class loaders. Currently we write all of these unregistered classes into the archive. However, only one copy of them will be usable at runtime.

For size saving, we should avoid writing such duplicated classes into the archive.

Note that this problem does not affect the CDS static dump, which is driven by a classlist file. We already disallow duplicate unregistered classes in the classlist.