base/tensor.h: work around bogus compiler warning

[tamiko]

gcc-9.4 warnings loudly about an unused argument "end":

In file included from ../include/deal.II/base/symmetric_tensor.h:26,
                 from ../include/deal.II/base/array_view.h:23,
                 from ../source/base/tensor.cc:16:
../include/deal.II/base/tensor.h: In instantiation of 'void dealii::Tensor<rank_, dim, Number>::unroll(Iterator, Iterator) const [with Iterator = float*; int rank_ = 1; int dim = 1; Number = float]':
../include/deal.II/base/tensor.h:1736:11:   required from 'void dealii::Tensor<rank_, dim, Number>::unroll(Iterator, Iterator) const [with Iterator = float*; int rank_ = 2; int dim = 1; Number = float]'
../source/base/tensor.cc:52:5:   required from 'void dealii::{anonymous}::calculate_svd_in_place(dealii::Tensor<2, dim, Number>&, dealii::Tensor<2, dim, Number>&) [with int dim = 1; Number = float]'
../source/base/tensor.cc:85:25:   required from 'dealii::Tensor<2, dim, Number> dealii::project_onto_orthogonal_tensors(const dealii::Tensor<2, dim, Number>&) [with int dim = 1; Number = float]'
../source/base/tensor.cc:92:60:   required from here
../include/deal.II/base/tensor.h:1728:51: warning: parameter 'end' set but not used [-Wunused-but-set-parameter]
 1728 |                                    const Iterator end) const
      |

This is of course bogus, the compiler simply lost track of the if constexpr(...) branch. This is fixed in later gcc versions. Let's simply use the usual (void)parameter strategy to make gcc-9.4 happy.

In reference to #16491 (for tracking purposes).
Closes #16491

[tamiko]

@bangerth fyi.

[bangerth]

Thanks!