DERBY-5840: Clean up compiler warnings introduced by using Java 5 language features

Generify CollectNodesVisitor so that the SuppressWarnings annotation
on its visit() method can be removed.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1485961 13f79535-47bb-0310-9956-ffa450edef68