RFR: 8049426: Minor cleanups after G1 class unloading

[Erik Helin]

Hi all,

this change cleans up a few comments and one assert message introduced
in the G1 class unloading patch.

Webrev:
http://cr.openjdk.java.net/~ehelin/8049426/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8049426

Thanks,
Erik