NEXUS-7769: Step 1: Implode LDAP plugin

As when we merged LDAP OSS and Pro, we left all the packages
and code mainly untouched with a lot of redundancy.

Changes:
* normalized plugin code under org.sonatype (before it was com and org)
* removed OSS config, hence OSS-Pro upgrade step too
* only Pro modello left in place for now
* removed unused and redunant classes

Issue
https://issues.sonatype.org/browse/NEXUS-7769

CI
http://bamboo.s/browse/NX-OSSF447
We'll need to remember to handle this change when migrating legacy nexus.xml, as it will refer to the old realm name.