Drop Client::cap_deco as it's nowhere used

Summary:
cap_deco was only used in the caption with stripped arg case which was
unused and thus removed. Now cap_deco is completely unused.

Due to that we can also remove the stripped client script which only
manipulated the cap_deco.

Test Plan: Compiles

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7077