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

Remove two unused methods in the TriggerExecutionContext interface,
and their implementations in InternalTriggerExecutionContext. This
also made it possible to remove some fields that were only used by the
removed methods, and also corresponding fields and parameters in
neighbour classes.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1477477 13f79535-47bb-0310-9956-ffa450edef68