de7b8dd23c97 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 de7b8dd23c97
there were doubts about the general usefulness.