Removes Node.unsafe_get_flags().

`Node.unsafe_get_flags()` returns `*const NodeFlags`,
but `NodeFlags` has only `u8` length.
We should just returns a raw value instead of any pointers.
auto merge of #3694 : saneyuki/servo/node, r=jdm

`Node.unsafe_get_flags()` returns `*const NodeFlags`, but `NodeFlags` has only `u8` length.
So We should just returns a raw value instead of any pointers.