Remove obsolete add_dependencies

Summary:
History suggests this originated in the kde-workspace repo. Now that
khotkeys and kmenuedit are split, kmenuedit doesn't know any
kded_khotkeys target anymore, but find_packages() it as an external
dependency.
Gets rid of this CMake warning: "Policy CMP0046 is not set: Error on
non-existent dependency in add_dependencies."

Test Plan: warning vanishes, builds successfully

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5699