DERBY-534: Support use of the WHEN clause in CREATE TRIGGER statements

Remove logic in the readExternal() and writeExternal() methods of
TriggerInfo and TriggerDescriptor that was originally put there for
compatibility between different Derby versions. Since these objects
are only persisted as part of a stored prepared statement, and Derby
always clears all stored prepared statements on version change, there
is no requirement that TriggerInfo and TriggerDescriptor instances
written by one Derby version must be possible to read by other Derby
versions.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1535654 13f79535-47bb-0310-9956-ffa450edef68