[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 had  actually kept this here as a placeholder, sort of as a hack, to make sure that there is no error when the loss is multinomial. (Liblinear does not solve multinomial problems). Now since both the solvers are independent, I removed it.