text: Title: Remove option `acq_optimizer="auto"`
  Body: In order to simplify  slightly the API, I would vote for removing the `"auto"` value of `acq_optimizer`. 
  
  I am not convinced we should default to random sampling as soon as one of the dimensions is not `Real`. 
  
  I would rather default to `"lbfgs"` and have, maybe, a warning if one the dimensions isnt `Real`. (Which by the way does not mean that the optimization will fail.)