DERBY-6169: Reduce visibility of classes and methods under impl/sql

Reduce visibility of the following methods, and all overrides of them,
to package:

- QueryTreeNode.generate()
- QueryTreeNode.getNullNode()
- ResultSetNode.bindResultColumns()
- ResultSetNode.generateNormalizationResultSet()
- ResultSetNode.generateResultSet()
- ResultSetNode.getFromTableByName()
- ValueNode.generateExpression()

Remove unused method:

- ValueNode.generateFilter()

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1480187 13f79535-47bb-0310-9956-ffa450edef68