CORE: fixed implementation of delete password method

 - delete password only when reserved by registrar
 - do not delete password when reserved by attribute or not reserved at all

 Previous wrong behavior

  - if not reserved by both attribute and registrar -> delete password
  - if reserved only by attribute -> throw exception
  - if reserved only by registrar or both -> do nothing
Merge pull request #253 from zlamalp/deletepassword

CORE: fixed implementation of delete password method