Unused dispersivity function in spatial params

The function dispersivity defined in

• https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/examples/1ptracer/spatialparams_tracer.hh#L78
• https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/test/porousmediumflow/1pnc/1p2c/spatialparams.hh#L87
• https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/test/porousmediumflow/tracer/constvel/spatialparams.hh#L78

is not used anywhere and should be deleted.