Revert "CMakeLists.txt: use ccache if present"

Summary:
This was added for feature-parity with Meson-based projects; but an
individual package is the wrong place for that. To avoid the proliferation
of auto-detected ccache support in kde.org packages please submit such
a proposal to kde-frameworks-devel instead, where it could be added for
the benefit of all the projects using ECM. And more importantly, with a
standard switch to disable it for packaging environments

This reverts commit 007c2a08.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: kkharlamov, hindenburg, konsole-devel, hiangel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D24805