Update cassandra stress benchmark to calculate max op rate and measure client cpu utilization.

PiperOrigin-RevId: 891914877

Author: Arushi-07

perfkitbenchmarker/configs/default_benchmark_config.yaml

@@ -689,7 +689,6 @@ kvrocks_memtier:
 cassandra_stress_mixed_workload:
   name: cassandra_stress
   flags:
-    cassandra_stress_run_duration: 10m
     cassandra_stress_mixed_ratio: 'write=3,read=1'
     cassandra_stress_command: 'mixed'