[kioslave/remote] Fix broken kded module

Summary:
Commit 72e39f08 broke the kded module. Turns out that the old code
just forced a refresh of the `remote:/` QUrl, so we can just do that
explicitly and remove all the old cruft.

While at it, use a KDirWatcher that just watches the
`.local/share/remote/` folder, instead of checking *every* unrelated
QUrl emitted by KDirNotify.

Test Plan:
Adding/deleting/editing .desktop files in `.local/share/remoteview` now
automatically reloads the `remote:/` url in Dolphin or Folder View.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4573