Don't search for KSane::Core in KF5SaneConfig
Currently KSaneCore is required to build against libksane because KF5SaneConfig.cmake.in asks for it.
However, src/CMakeLists.txt only lists it as private library, which means that it's only necessary when building libksane itself.
Which of those is correct? Currently KSaneCore is not referenced by the header files, but I imagine that might change in the future.