MINIFICPP-2753 - Remove restore from c-api

[adamdebreceni]

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[szaszm]

why?

[martinzink]

why?

I can answer that, this fn is only used for niche usages and kinda hacky (storing flowfiles in the self relationship for later use). This features is only used by 4 processors (MergeContent, DefragmentText, ListenHTTP, PushGrafanaLoki), and I feel like this could be done cleaner so its better not to commit to this architecture in the first release of the C API. (hopefully we can think of something better or if we cant we can easily add this later without breaking the API)