Code cleanup in pom files

Do not meld reactor pom (opendj-sdk-bom) which should be used to declare
module to be build and parent pom.
The only effective difference introduced by this change in the build
process is that the following command:
`cd opendj-sdk-parent && mvn clean install`
will only build opendj-sdk-parent module instead of all opendj-sdk
modules before.
Thanks to Guillaume for the tip.