AutoLoader: Remove "Class ... not found" debug message

Per existing FIXME comment: "This is not very polite. Assume we
do not manage the class."

Reverts the remaining portion of r44296 / 2f584f68a95a while
clarifying the "Give up" comment.

Change-Id: I4471f9376cae6b3c3e8df99e960770863ebf37ca
Merge "AutoLoader: Remove "Class ... not found" debug message"