`DirectionalTargetDerivative` is untested

[inducer]

I don't think it works any more, gathering of target arguments seems like it has bitrotted. It's possible to get an equivalent result by computing a gradient and dotting it with the normal, that's what pytential does.

cc @ShawnL00

[inducer]

It might be better to remove it entirely, to avoid confusion?

[isuruf]

Yeah, I don't think it's used in pytential (even in my giant stokes PR)