[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.
The whole thing is very nested: it's hard to see which `else` belongs with which `if`. It's the depth of nesting which causes the line-wrapping, which causes the length ...