Remove unused arguments from np_datetime_strings
Same deal as #18546.  This completes the pure-cleaning phase for np_datetime_strings.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=h1) Report
> Merging [#18565](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7627ccaf9442f4101afda69b6077e7f035e23543?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/18565/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18565      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.02%     
==========================================
  Files         164      164              
  Lines       49802    49802              
==========================================
- Hits        45496    45487       -9     
- Misses       4306     4315       +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/18565?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565?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/18565?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/18565?src=pr&el=footer). Last update [7627cca...419e02c](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=h1) Report
> Merging [#18565](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a7646638d06f1ce98481b88f3505e2b4badf172c?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/18565/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18565      +/-   ##
==========================================
- Coverage   91.59%   91.58%   -0.02%     
==========================================
  Files         155      153       -2     
  Lines       51255    51253       -2     
==========================================
- Hits        46948    46941       -7     
- Misses       4307     4312       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.45% <ø> (ø)` | :arrow_up: |
| #single | `40.67% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/data.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2RhdGEucHk=) | | |
| [pandas/io/wb.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3diLnB5) | | |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18565/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.01% <0%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18565?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/18565?src=pr&el=footer). Last update [a764663...6af5366](https://codecov.io/gh/pandas-dev/pandas/pull/18565?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 After this and #18567 there are two more PRs on deck to simplify src/datetime. yep rebase Travis error is a timeout, looks unrelated.  Will re-push this evening. lgtm. ping on green. ping Once this is closed we can kill off datetime.pxd kill it!