Remove duplicate GaussianNB.fit() code

Follow-on to #3324. 

`GaussianNB.fit` is substantially duplicated by `partial_fit`, and the latter is more flexible, so just have the former call the latter. There should be no significant degradation in performance or numerical stability, since the mean/variance update will just call `numpy.mean` and `numpy.var` when `n_past == 0` rather than trying anything fancy.