CLN: remove obsolete code in Categorical.fillna
closes #19787  

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
Deleted code and tested block of code associated with NaN that should be unreachable. 
Thanks. Looks like some test cache files got in your commits.

Could you

1. Delete those files: `rm -rf .pytest_cache`
2. Remove them from the next commit (I think `git rm .pytests_cache`
3. Add `.pytest_cache` to the `.gitignore` file at the root of the pandas directory?

And then push those changes. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19880?src=pr&el=h1) Report
> Merging [#19880](https://codecov.io/gh/pandas-dev/pandas/pull/19880?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/38afa9310040f1bd4fb122008e96fe6d719b12a2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19880/graphs/tree.svg?height=150&token=eZ4WkYLtcO&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19880?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19880      +/-   ##
==========================================
+ Coverage   91.78%   91.78%   +<.01%     
==========================================
  Files         152      152              
  Lines       49196    49191       -5     
==========================================
- Hits        45153    45151       -2     
+ Misses       4043     4040       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.17% <ø> (ø)` | :arrow_up: |
| #single | `41.82% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19880?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/19880/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.47% <ø> (+0.36%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19880?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/19880?src=pr&el=footer). Last update [38afa93...a1df599](https://codecov.io/gh/pandas-dev/pandas/pull/19880?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!