[MRG] MAINT: Remove BaseLibLinear for LogisticRegression

BaseLibLinear has very little to do with liblinear in master.

This replaces it with a private `_fit_liblinear` function and simplifies the recursive code paths to an extent.
I much prefer this less-nested structure, but it still takes substantial effort for the reader to realise that the only differences between this call and the one for multinomial are `multi_class` and `pos_class` (and `copy`, but passing `copy=True` is unnecessary as it is the default). Instead one could just set `classes_ = [None]` in the multinomial case.

This is also true of the LogisticRegressionCV implementation.