drop --output-format=json+ from block throughput tests

this option only affects the output to stdout, but we are ignoring fio's
stdout (we only work with the log files, which are separate). So drop
this parameter.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/integration_tests/performance/test_block_ab.py

@@ -69,7 +69,6 @@ def run_fio(microvm, mode, block_size):
         .with_arg("--cpus_allowed_policy=split")
         .with_arg(f"--write_bw_log={mode}")
         .with_arg("--log_avg_msec=1000")
-        .with_arg("--output-format=json+")
         .build()
     )