CLN: dedent else part of Series._get_with
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

There is a gigantic and unneeded ``else`` clause in ``Series._get_with`` and it makes the code hard to read.

This PR removes the else clause and dedents its content one level. This helps readability.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22406?src=pr&el=h1) Report
> Merging [#22406](https://codecov.io/gh/pandas-dev/pandas/pull/22406?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9f6c02dd40e3ccd4dc13dcc2ab449b7aed2dc4ae?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `85.71%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22406/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22406?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22406   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         169      169           
  Lines       50709    50709           
=======================================
  Hits        46679    46679           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <85.71%> (ø)` | :arrow_up: |
| #single | `42.25% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22406?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/22406/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.72% <85.71%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22406?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/22406?src=pr&el=footer). Last update [9f6c02d...89fadb0](https://codecov.io/gh/pandas-dev/pandas/pull/22406?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks