make scripted sleep no-op

Summary:
this was implemented using a nested loop, which is simply not viable
because nested looping may work on inconsistent object states and cause
crashes all over the place. furthermore sleeping probably has no real use
case as there isn't anything you could be sleeping for

prevent any future problems by making the API method do nothing and instead
print a warning about it being deprecated.

should be removed when moving over to kf6

Test Plan: prints warning when sleeping

Reviewers: davidedmundson, mart

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22070