Remove shell's copy of PlasmaQuick headers

Summary:
It's exported and we're including the include path in our cmake so we
had two paths to the same thing included.

Test Plan: Compiled

Reviewers: #plasma, markg

Reviewed By: markg

Subscribers: markg, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4030