CLN: liboffsets de-duplicate pickle code
privatize/cdef functions that are now only used in liboffsets
@jbrockmendel if might be worth making another pickle file on 0.25.3 (and pushing to master) with all of the supported dateoffsets just to ensure we didn't break anything (unless they are already there). e.g. https://github.com/pandas-dev/pandas/blob/master/pandas/tests/io/generate_legacy_storage_files.py (though it looks like we most/all of these already)