Remove cmake/modules/ and cmake/CMakeLists.txt

cmake/modules only had yet another copy of FindIcecream.cmake, and besides that the file was being installed, which is not good (Find*.cmake files should not be installed by the applications themselves).

Furthermore, CMAKE_MODULE_PATH is not being set in the top-level CMakeLists.txt file anymore.
As discussed today on #kde-devel, doing so meant that we considered the applications in playground part of a coherent bundle of apps, such as kdenetwork or kdesdk.

This is not the case, as each application is here only because they are still under heavy development; the situation is more similar to extragear, in which each application does its own release separately.