Remove unused/unreachable code from ops
Add TODO comments for questionable cases.

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18964?src=pr&el=h1) Report
> Merging [#18964](https://codecov.io/gh/pandas-dev/pandas/pull/18964?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c19bdc98fa3f856e2e6b3495bbab88b587801987?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18964/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18964?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18964      +/-   ##
==========================================
+ Coverage   91.57%   91.59%   +0.02%     
==========================================
  Files         150      150              
  Lines       48941    48909      -32     
==========================================
- Hits        44817    44798      -19     
+ Misses       4124     4111      -13
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.95% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.77% <90%> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18964?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/18964/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `93.94% <100%> (+3.7%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18964/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/computation/expressions.py](https://codecov.io/gh/pandas-dev/pandas/pull/18964/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwcmVzc2lvbnMucHk=) | `93.27% <0%> (-1.69%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/18964/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.04% <0%> (-0.02%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18964/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.95% <0%> (+0.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18964?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/18964?src=pr&el=footer). Last update [c19bdc9...5b704fa](https://codecov.io/gh/pandas-dev/pandas/pull/18964?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @jbrockmendel! Thanks for updating the PR.







Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on January 01, 2018 at 16:47 Hours UTC closing in favor of #19024 