RFR: 8198447: Remove ClassLoaderExt::check()

[Jiangli Zhou]

Please review the code cleanup for JDK-8198447: Remove ClassLoaderExt::check().

webrev: http://cr.openjdk.java.net/~jiangli/8198447/webrev.00/
bug:      https://bugs.openjdk.java.net/browse/JDK-8198447

ClassLoaderExt::check() does:
 - Check if a class is from signed JAR and skip archiving if so
 - Set _has_app_classes & _has_platform_classes to true if the current class is app class

ClassLoaderExt::check() was only called for the boot loader. This function was needed to handle app classes when the boot loader was used to load all boot/platform/app classes at CDS dump time. Starting from JDK 10, platform & app classes are no longer loaded by the boot loader at dump time. So ClassLoaderExt::check() is no longer needed and should be removed.

Thanks,
Jiangli