Undo custom support for ccache

de7b8dd2 added custom CMake code to support compiling with `ccache`,
however with current CMake versions (since 3.4, which even Ubuntu 16.04
ships) one can simply use `-DCMAKE_CXX_COMPILER_LAUNCHER=ccache`.

Let's simplify Gwenview's CMake code. If at all, such a feature should
end up in ECM, but even in the discussions in D7288 and de7b8dd2
there were doubts about the general usefulness.