replace enable_if in template for rank check by `xt::get_rank`

GNU GCC was complaining about a missing type definition ("no type name
'type' in 'struct::enable_if<false, int>'") for the false case
(as explained here https://stackoverflow.com/a/12625334). So given that
there are two/three false conditions for `evald`/`evalp`, it seemed
better to drop these rank checks from the templates, and use the
function `xt::get_rank` instead.