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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=h1) Report
> Merging [#18533](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/88ab6934e78117359719eb09c7d580906155575d?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18533/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18533      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.02%     
==========================================
  Files         164      164              
  Lines       49801    49801              
==========================================
- Hits        45494    45485       -9     
- Misses       4307     4316       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <ø> (ø)` | :arrow_up: |
| #single | `40.81% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=footer). Last update [88ab693...764009c](https://codecov.io/gh/pandas-dev/pandas/pull/18533?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 do u have linting settings that show these?
if so they should be put up as well > do u have linting settings that show these?

You mean show that they are unused?  I didn't know linting could do that.  No, this is just based on grepping and seeing only the definition. yes might be a way to do this more automatically If you find such a way let me know.  Until then, any objection to PRing them manually as I find them? yeah sometimes things build up. thanks! > 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 t

yeah these were put in here for lack of a better location. we *could* add a separate small lib for this to reduce lib.pyx size if it helps.