LdapUrl: remove ctor code which made the QUrl invalid.

Summary:
dn() already takes care of removing the leading slash from the path
anyway.

This only showed up during debugging of a new unittest, because actual
kioslave code (which is where LdapUrl is built from QUrl) only queries
for components out of the LdapUrl, not for the full string
representation.

Test Plan: unittest in next commit

Reviewers: dvratil, mlaurent

Reviewed By: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12386