Revise plugin documentation. Mainly, this removes remains of documentation

Revise plugin documentation. Mainly, this removes remains of documentation
that's only relevant for plugins which are not included in the main distro.
The objective is to make it not
- remove instructions about untarring this plugin (since it's already shipped)
- remove version numbers, which are not updated for a long time and not
  relevant anymore.
- remove changelogs, since they don't log the changes (we do that in the
  main ChangeLog).
- explicitly state that this plugin is not maintained anymore by the
  original authors (so bugs/etc get sent to us in stead of someone who
  maintained it four years ago).

If ok I'd like to put this in stable aswell.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@8921 7612ce4b-ef26-0410-bec9-ea0150e637f0