remove --memstats from cfg_test

remove --memstats from cfg_test

The `--memstats` option from cfg_test is unused, and even if used, does
nothing because `--memstats` relies on `isc_mem_stats` which dump memory
pools statistics, which are not used at all for configuration.

Also, dropping the option avoid to add a parser API to get the memory
stats (as the parser now uses the global memory context).