Remove constructor Quadrature(unsigned int).

[bangerth]

My exchange student wanted QGauss<dim>(3), but accidentally did Quadrature<dim>(3). This creates a quadrature formula with three points, all at the origin, and weight zero. Perhaps not surprisingly, in hindsight, all his integrals were zero. It was not what he wanted.

I think we should remove this constructor, or at least make it protected. It doesn't create a useful object, and leads to hard-to-spot mistakes.