ruby: simplify data get helper

ruby: simplify data get helper

Data_Get_Struct is nothing but a macro that calls
rb_data_object_get with a cast (unnecessary in C).

        #define Data_Get_Struct(obj, type, sval) \
            ((sval) = RBIMPL_CAST((type*)rb_data_object_get(obj)))

We can use rb_data_object_get directly, and this way we don't need to
pass the type, which is unnecessary information.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>