Catch up to SinglePhaseUpscaler::init() interface change

Commit OPM/opm-upscaling@886c463 removed the `z_tol` parameter from method `SinglePhaseUpscaler::init()`, preferring to retrieve the value from the `PINCH` keyword or defaulting to zero if `PINCH` was not specified.  This did not result in a build failure in module opm-benchmarks due to additional, defaulted method parameters in the `init()` method.  It did however lead to interpreting the `ztol` setting of `upscale_relperm_benchmark.cpp` as the (absolute) linear residual tolerance.

That in turn, because `ztol=0`, resulted in the linear solver never actually converging to an acceptable solution according to the specified criteria and rather ending up taking the maximum number of linear iterations (= system dimension), effectively increasing the computational cost by a factor of about two.

This commit removes the `ztol` parameter to restore the original behaviour.