CLN: Remove io/formats/terminal.py
Remaining is_terminal function is used in `core/config_init.py`, and as this function is not meant to be public, we can move it there.

This will help with dependency organization of pandas' modules.

Follow-up to #25886.

cc @jbrockmendel 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=h1) Report
> Merging [#25916](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7721f7009249e2b1fa2647526114e7c423fef4ea?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `62.5%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25916/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25916      +/-   ##
==========================================
- Coverage   91.77%   91.76%   -0.01%     
==========================================
  Files         175      174       -1     
  Lines       52606    52604       -2     
==========================================
- Hits        48280    48274       -6     
- Misses       4326     4330       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <62.5%> (-0.01%)` | :arrow_down: |
| #single | `41.9% <62.5%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/25916/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.96% <62.5%> (-2.24%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25916/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25916/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25916?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/25916?src=pr&el=footer). Last update [7721f70...4f3d078](https://codecov.io/gh/pandas-dev/pandas/pull/25916?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/25916?src=pr&el=h1) Report
> Merging [#25916](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7721f7009249e2b1fa2647526114e7c423fef4ea?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `62.5%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25916/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25916      +/-   ##
==========================================
- Coverage   91.77%   91.76%   -0.01%     
==========================================
  Files         175      174       -1     
  Lines       52606    52604       -2     
==========================================
- Hits        48280    48274       -6     
- Misses       4326     4330       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <62.5%> (-0.01%)` | :arrow_down: |
| #single | `41.9% <62.5%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/25916/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.96% <62.5%> (-2.24%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25916/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25916/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25916?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/25916?src=pr&el=footer). Last update [7721f70...4f3d078](https://codecov.io/gh/pandas-dev/pandas/pull/25916?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung 