[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'd rather have the following error message "This solver needs samples of at least 2 classes in the data, but the data contains only one class: %r" % classes_[0].