Do not save JSON summaries by default

[wjxiz1992]

If --json_summary_folder arg is not specified, the JSON summary files will be saved to the default folder "json-summary" by default.
When we run the test for the second time, we detect if the json_summary folder has any files inside, if so, the code will raise an exception to protect those existed summaries.
Some users complain it's really annoying and they don't need those summary files.
Also when running Throughput Test, if this json_summary argument is not specified differently, the run will fail due to duplicated summary folder name.

[GaryShen2008]

Since our CI jobs specify --json_summary_folder always, we can simply modify the script that don't save JSON summary when not setting --json_summary_folder. It won't impact the CI.