DERBY-6087 Can't build Javadoc with JDK 1.8 if explicit j15lib, j16lib and j17lib paths are set

Simple fix which make the javadoc build use java18compile.classpath.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1449437 13f79535-47bb-0310-9956-ffa450edef68