DERBY-6371: DROP COLUMN looks in wrong schema when checking trigger dependencies

Fix another occurrence where the wrong schema is used. This occurrence
doesn't cause any problems currently, since it's only used for
parsing, and the problems wouldn't appear until one attempt to do
binding with the incorrect schema. Fix it anyway for consistency.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1530913 13f79535-47bb-0310-9956-ffa450edef68