Minor cleanup: consistent deserialization.

Merge request !185 (merged) eliminated the need to define a gsonDeserializerOf implementation.

Moreover, the repositories inconsistently imported a gsonDeserializerOf from either audio.funkwhale.ffa.utils or com.hitmub.kittinunf.fuel.gson.

This simply establishes a consistent dependency on com.github.kittinunf.fuel.gson.