Move kcheckrunning into startkde

Summary:
kcheckrunning is only used by startkde, it shouldn't be in the ksmserver
folder.

No code or behavioural changes, just tidying.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15202