Unused reference solutions

These reference solutions are currently not used:

1p2ctestccmpfa-reference.vtu
1pnctestbox-00009.vtu
1pnctestcc-00009.vtu
1ptestccmpfa-reference.vtu
2cnistokes2p2cniboundarylayer-ff-reference.vtu
2cnistokes2p2cni-ff-reference.vtu
2cnistokes2p2cni-pm-reference.vtu
2cnizeroeq2p2cni-ff-reference.vtu
2cstokes2p2c-ff-reference.vtu
2czeroeq2p2c-ff-reference.vtu
2pdfm-reference.vtu
box1pncmin-00042.vtu
box2pmincdist-reference.vtu
box2pmincvol-reference.vtu
cc1pncmin-00042.vtu
ccmpfatracer-reference.vtu
el1p2c-reference.vtu
el2p-parallel-reference.vtu
el2p-reference.vtu
elasticmatrix-reference.vtu
forchheimer1p-reference.vtp
forchheimer2p-reference.vtu
generalizeddirichlet-reference.vtp
generallens_box-reference.vtu
generallens_cc-reference.vtu
generallens_sequential-reference.vtu
rosi2c-root-reference.vtp
rosi2c-soil-reference.vtu
test_adaptive2p2c2d-reference.vtu
test_adaptive2p2c3d-reference.vtu
test_ff_navierstokes_sincos_stationary-reference.vtu
test_md_boundary_darcy2p2cni_stokes1p2cni_vertical_darcy-reference.vtu
test_md_boundary_darcy2p2cni_stokes1p2cni_vertical_stokes-reference.vtu
test_md_boundary_darcy2p2c_stokes1p2c_vertical_darcy-reference.vtu
test_md_boundary_darcy2p2c_stokes1p2c_vertical_stokes-reference.vtu

If nobody objects in general or against individual files before July 21, I will file a merge request that deletes the (remaining) files.