Lib_openapi: remove unused Json value from Api.arg

Context

This MR removes the unused field "json" of type Json.t from type Api.arg in lib_openapi. This value contained a Json representation of the argument, which was no longer used.

This has the additional benefit of removing the spurious expected path /foo/bar but found /foo/bar warning messages that happened when runnning the tests un qcheck_rpc.ml. Indeed, those only happened because two paths had two args with two json values that were slightly different.

Part of #3281 (closed)

Manually testing the MR

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