cmake: remove MacroOptionalFindPackage
This macro is no longer needed:
Starting with CMake 2.8.6, every find_package() call which is not
REQUIRED can be disabled by setting the variable
CMAKE_DISABLE_FIND_PACKAGE_<PackageName> to TRUE
See also:
https://techbase.kde.org/ECM5/IncompatibleChangesKDELibs4ToECM#MacroOptionalFindPackage.cmake