Undo custom support for ccache
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.