Removed unused funcs from _libs
See coverage report: https://github.com/pandas-dev/pandas/pull/18512#issuecomment-347281660

This just removes a handful of unused functions.  LMK if some should be kept around just-in-case.

Not included in the never-used group but worth mentioning: lib.fast_unique, lib.convert_timestamps, and lib.string_array_replace_from_nan_rep are each called exactly once in io.pytables, not covered in tests (io.pytables looks poorly covered, might have had a bunch of skipTests locally)

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
