RFR (trivial) 8209792: Remove ClassLoaderDataGraph::keep_alive_cld_do

[coleen.phillimore at oracle.com]

Summary: unnecessary

Built without it, ran runThese -jck tests (with lots of class unloading).

open webrev at http://cr.openjdk.java.net/~coleenp/8209792.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8209792

Thanks,
Coleen