`taxa` column does not work with `dplyr::distinct`

[brycefrank]

In v1, when we checked if model specifications were unique we relied on dplyr::distinct to determine this. However, dplyr::distinct does not work with S4 objects. It may be possible to write an S4 method for Taxa that might allow us to determine distinct elements across multiple taxa (e.g., in the taxa column of a model specification). I have opened a stack overflow question asking after this.

For now we will remove the check_model_specifications_unique from FixedEffectsSet (apparently it was not implemented in MixedEffectsSet anyway) until we come to a better solution. At worst, this places us at risk of creating duplicate models in sets, but this was not a big issue anyway.