CLN: Remove unnecessary io function returns
These functions return a function either with empty return statement or doesn’t return. It causes docstring validation error with type RT01 (See #26234).

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=h1) Report
> Merging [#26238](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9feb3ad92cc0397a04b665803a49299ee7aa1037?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26238/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26238      +/-   ##
==========================================
- Coverage   91.97%   91.96%   -0.01%     
==========================================
  Files         175      175              
  Lines       52368    52368              
==========================================
- Hits        48164    48161       -3     
- Misses       4204     4207       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <50%> (ø)` | :arrow_up: |
| #single | `40.69% <25%> (-0.16%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.54% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.71% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26238?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/26238?src=pr&el=footer). Last update [9feb3ad...45b5409](https://codecov.io/gh/pandas-dev/pandas/pull/26238?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/26238?src=pr&el=h1) Report
> Merging [#26238](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9feb3ad92cc0397a04b665803a49299ee7aa1037?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `40%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26238/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26238      +/-   ##
==========================================
- Coverage   91.97%   91.96%   -0.01%     
==========================================
  Files         175      175              
  Lines       52368    52368              
==========================================
- Hits        48164    48160       -4     
- Misses       4204     4208       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <40%> (ø)` | :arrow_up: |
| #single | `40.69% <20%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26238/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.54% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26238?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/26238?src=pr&el=footer). Last update [9feb3ad...bf4438b](https://codecov.io/gh/pandas-dev/pandas/pull/26238?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @ihsansecer 