drop RRA_STEP from process-perfdata.cfg

* RRA_STEP is set in both, process_perfdata.cfg-sample.in and rra.cfg-sample. There is further a hard-coded default in process_perfdata.pl.in.
* The value from rra.cfg-sample seems to override that of process_perfdata.cfg-sample.in.
* Given that IMHO, the RRD/RRA settings should go to rra.cfg and given that there’s a hard-coded default anyway, I dropped RRA_STEP from the config.
  This does not drop the code for reading that value from process-perfdata.cfg from process_perfdata.pl.in, although we should do that probably too.