Don't show knetattach in the menu, it's only useful from within a file manager/dialog

Summary:
knetattach is used by the remote:/ kioslave as a UI to add a new entry to a remote folder, it can be called (or should be this seems to have broken to some extent) from within Dolphin or within a file open/save dialogue. It's not useful to call it from the app menu as it just disappears and shows to context to the user

Test Plan:
install and check running from dolphin, file open/close and not from kickoff menu

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: broulik, andreaska, plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1539