DOC: temporary remove pyarrow example of reading subset columns
xref #18628.

Until pyarrow 0.8 is released, I propose to just show the example with fastparquet. Once pyarrow 0.8 is released, we can revert this change.




# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=h1) Report
> Merging [#18661](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fdba133336c2ec8f1269eb15e73d48797840c48b?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/18661/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18661      +/-   ##
==========================================
+ Coverage   91.57%   91.57%   +<.01%     
==========================================
  Files         153      153              
  Lines       51210    51210              
==========================================
+ Hits        46894    46897       +3     
+ Misses       4316     4313       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.43% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.67% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18661/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18661/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.25% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18661?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/18661?src=pr&el=footer). Last update [fdba133...18002a0](https://codecov.io/gh/pandas-dev/pandas/pull/18661?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/18661?src=pr&el=h1) Report
> Merging [#18661](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fdba133336c2ec8f1269eb15e73d48797840c48b?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/18661/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18661      +/-   ##
==========================================
+ Coverage   91.57%   91.57%   +<.01%     
==========================================
  Files         153      153              
  Lines       51210    51210              
==========================================
+ Hits        46894    46897       +3     
+ Misses       4316     4313       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.43% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.67% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18661/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18661/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.25% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18661?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/18661?src=pr&el=footer). Last update [fdba133...18002a0](https://codecov.io/gh/pandas-dev/pandas/pull/18661?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! should we open an issue to revert this (at the appropriate time)? We can keep #18628 open for that