[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'm thinking on how to factor out the `logistic_regression_path` call since it gets called `n_classes` for the `if` with n different options for the `pos_class` argument for `logistic_regression_path` and only once for the `else` option.