Remove custom serializer from association options

As of SHA: c8cfe94f299619382d2178344e99d6c6d0e02cb1, we now pass the `options` hash for one serializer down the associations. However, that also means we're now passing along the `:serializer` and `:each_serializer` options specified on a `has_one` or `has_many` association. This change will delete those values from the options hash when initializing the respective `Association` object.