[CMakeLists] Remove unused DARWIN_ICU_INCLUDE_PATH
[CMakeLists] Remove unused DARWIN_ICU_INCLUDE_PATH

This CMake variable was introduced in f6224bd3f, but its only usage was
subsequently removed in d9bbb6caf. Remove the obsolete variable to
avoid confusion.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift