Tensor polynomials: Remove nonsensical comments

[kronbichler]

Looking at #15913, I realized that all classes derived from TensorPolynomialsBase refer to some compute_value, compute_grad, compute_grad_grad functions that are not even present in these classes. I guess this is a copy-paste effect from the scalar tensor product polynomial class

dealii/include/deal.II/base/tensor_product_polynomials.h

Lines 126 to 129 in e3b4ff4

	* If you need values or derivatives of all tensor product polynomials then
	* use this function, rather than using any of the compute_value(),
	* compute_grad() or compute_grad_grad() functions, see below, in a loop
	* over all tensor product polynomials.

that actually has these functions

dealii/include/deal.II/base/tensor_product_polynomials.h

Lines 151 to 152 in e3b4ff4

	double
	compute_value(const unsigned int i, const Point<dim> &p) const override;

etc. While I would be tempted to introduce such a function to evaluate a single basis function at a point at least for the class PolynomialsRaviartThomas to get the matrix-free initialization reasonably fast (the current method has a quadratic complexity in ShapeInfo::reinit() because it evaluates the complete basis for every basis function we request via FE_PolyTensor::shape_value_component()), that is a wrong assumption, and it is in fact better to work towards #9655.