use envoy.reloadable_features.remove_legacy_json deprecation

[alyssawilk]

Your change #14467 (Replace rapidjson with nlohmann/json) introduced a runtime guarded feature. It has been 6 months since the new code has been exercised by default, so it's time to remove the old code path. This issue tracks source code cleanup so we don't forget.

[asraa]

@dio do you know if anyone has been running this with the flag flipped to true?

[alyssawilk]

ping!

[dio]

@asraa sorry for the very late reply, I don't think I know one. But seems like it is safe to remove this flag now?

[alyssawilk]

@dio the flag is false by default: the script picked it up because it's been in the code base but never flipped true.
@asraa thought you folks were testing with the flag flipped true, is that the case?
@asraa what do you think of just flipping it?

[dio]

the flag is false by default

@alyssawilk oops sorry too fast to comment. Will ask around.

[dio]

@wbpcode could you help to test this, since it seems you have a sensible environment to test this given: #4705 (comment). Thanks!

[dio]

Talked to @ramaraochavali from Salesforce offline, it seems he can help to test this in their environment. Thanks, @ramaraochavali!

[ramaraochavali]

@dio we deployed with that run time flag and basic flows (Service -> Service) communication are working

[wbpcode]

@dio Sorry for the delayed reply. I can also do a quick test in my local env with config of the production environment.