webdav docs: remove broken link
For post 5.101-tagging due to string freeze.
Broken in two ways:
- link was not updated to new scheme (would be now kioslave5/webdav/index.html)
- linking to itself which does not really make sense (like that from the start in 2002, see https://websvn.kde.org/trunk/kdebase/doc/kioslave/webdav.docbook?view=markup&pathrev=132880)