Revert "oxcmsg: Implementation for partial UserDNs in ModifyRecipients call"

This reverts commit ea9cf1980a13786ecdf17e710eaf256bee5f1152.

Unfortunately this commit brakes SOGo backend expectations that
recipient->username actually holds samAccountName for the user.
Thus although Openchange layer is doing the right thing now,
SOGo is not handling this very well. I need to work more on that.

Signed-off-by: Kamen Mazdrashki <kmazdrashki@zentyal.com>
Merge pull request #62 from Zentyal/kamenim/sz-3.2-push

Revert "oxcmsg: Implementation for partial UserDNs in ModifyRecipients call