Fix deletion of files from DAV

Summary:
Don't append slash to the end of what we assume is a directory path as
in the case of HTTP we can't always tell for sure if it's a file or a
directory.

This partially restores change 1bdb286f from Dawit.

BUG: 355441
FIXED-IN: 5.51

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15697