Use ibmint instead of requiring Maven for Java build

[trevor-dolby-at-ibm-com]

ACE v12 provides Java build capability with the ibmint command, and this could be used instead of Maven to avoid having to build a separate ace-minimal-build container image. The original ACE v11 pipeline had to use Maven to avoid pulling in mqsicreatebar for the Java build, but this is no longer the case with ACE v12.

Maven would still be useful for pulling in the DB2 JDBC JAR file (currently stored in this repo), but the pipeline setup would be greatly simplified by avoiding Maven entirely . . .