REF: prelim for fixing array_to_datetime
`tslib.array_to_datetime` has an unused `format` kwarg.  This gets rid of it.

`to_datetime` (or more specifically, `_convert_listlike_datetimes`) has a bunch of boxing logic inside a try/except that doesn't need to be, obscures what Exception we're trying to catch.  This moves that code outside the try/except and de-indents it.  This overlaps with #23675.

xref #23722 (the conversation there less than the OP)  cc @mroeschke 
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/tools/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/03e9ea8c57683bfeedcaf757982dc00c9bcf7d77/pandas/core/tools/datetimes.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24000?src=pr&el=h1) Report
> Merging [#24000](https://codecov.io/gh/pandas-dev/pandas/pull/24000?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7653a6b43a8a302bc1d88903c8c99730a00bcc80?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24000/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24000?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24000      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         161      161              
  Lines       51555    51557       +2     
==========================================
+ Hits        47589    47591       +2     
  Misses       3966     3966
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <100%> (ø)` | :arrow_up: |
| #single | `42.44% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24000?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `85.53% <100%> (+0.08%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24000?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/24000?src=pr&el=footer). Last update [7653a6b...03e9ea8](https://codecov.io/gh/pandas-dev/pandas/pull/24000?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks