phpdocumentor is not needed in Elena any more

phpdocumentor is not needed in Elena any more

... as it causes lots of problems, like this:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for phpdocumentor/unified-asset-installer 1.1.2
  -> satisfiable by phpdocumentor/unified-asset-installer[1.1.2].
    - phpdocumentor/unified-asset-installer 1.1.2 requires composer-plugin-api 1.0.0
  -> no matching package found.

Change-Id: Iafbcc83e0922fac37494924d8bdd328f3d81be72
Reviewed-on: http://gerrit.tine20.com/customers/3303
Reviewed-by: Philipp Schüle <p.schuele@metaways.de>
Tested-by: Philipp Schüle <p.schuele@metaways.de>