[kcmkwin/compositing] Hide animation speed slider on Plasma

Summary:
Hide the animation speed slider on Plasma as this becomes duplicated
with the global "animation speed" slider.

Also port to the new format for the config option which stores the
actual multiplier, not a magic number.

Test Plan:
Didn't appear normally
Manually modified env; slider appeared
Tested load/save of defaults and the two extremes

Reviewers: zzag

Reviewed By: zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24075