[ksmserver] Remove blocking call introspecting KDED

Summary:
We don't need to introspect at runtime as KDED installs the interface
XML file.

Oddly KDED's package is called KDED not KF5KDED, hence the separate
find_package, I don't know if that's a bug in frameworks or not.

Test Plan: Compiles

Reviewers: #plasma, romangg

Reviewed By: #plasma, romangg

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16628