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:
* 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
TBD