Trim unncessary code in datetime/np_datetime.c
`pydatetime_to_datetimestruct` does a ton of checking that boils down to "is this a valid datetime object?"  Since the function only gets called after a type-check, we can assume it is a date/datetime and be a lot less verbose about it.

This also rips out an unnecessary layer of functions `convert_datetime_to_datetimestruct`, `convert_timedelta_to_timedeltastruct`.

cc @WillAyd you mentioned wanting to work on your C-foo.  There's a comment about figuring out how to import the cpython datetime C-API.  Any thoughts?
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21962?src=pr&el=h1) Report
> Merging [#21962](https://codecov.io/gh/pandas-dev/pandas/pull/21962?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4b73f22d0b8312607907515395266eea774431bf?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21962/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21962?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21962   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50329    50329           
=======================================
  Hits        46287    46287           
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21962?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/21962/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.46% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21962?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/21962?src=pr&el=footer). Last update [4b73f22...69a8459](https://codecov.io/gh/pandas-dev/pandas/pull/21962?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 