Fix kcm_ktp_chat_messages appearing in global search

The exec is not needed when we load it as a plugin and it is defunct anyways, because kcmshell4 does not work when using KF5 compiled KCMs.
Systemettings will skip KCMs in the deprecate dKServiceTypeTrader codepath that do not have an exec defined.


(cherry picked from commit f4df6eaf)