cext: make HgRevlogIndex_GetParents private again The rust's direct-ffi code need to access this function. Now that it `direct-ffi` is no more. We can make this function `static` again. This change was suggested by `Yuya Nishihara`.

setup-rust: remove the legacy 'direct-ffi' variant This variant have been abandoned for a while. Keeping it around just get people confused.