Refactoring targeting_config_slug to use targeting_config

[yashika-khurana]

We added targeting_config in the api end point- experimentBySlug and targeting_configs in getConfig Api end point, which includes field label, value, description and application values, so now we can get rid of targeting_config_slug field in the api, need to modify usage of this field on frontend, removing field from backend and fixing test cases (both frontend and backend)

PR where this new field added: https://github.com/mozilla/experimenter/pull/7228

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