DEP: deal with deprecation of ndim >1 in bspline

[h-vetinari]

Since 0131d72 from #3174, there's a warning that says

Calling splev() with BSpline objects with c.ndim > 1 is not recommended. Use BSpline.__call__(x) instead.

It does raise a DeprecationWarning (and for over 5 years, at that), but the message text is a bit soft. Either we should sharpen the deprecation warning to schedule removal in 1.10 / 1.11, or simply remove it for 1.9.

CC @ev-br

[ev-br]

Since there was not a single complaint about this, we can even raise an error. If anybody feels stong enough about it --- not sure whether it's of any use. It won't be less code, so as far as I'm concerned it can just be left as-is.