TST/CLN: Remove legacy instances of _multiprocess_can_split_ in tests
- [X] closes #19532
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

xref https://github.com/pandas-dev/pandas/pull/19509#discussion_r165848506
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19536?src=pr&el=h1) Report
> Merging [#19536](https://codecov.io/gh/pandas-dev/pandas/pull/19536?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bc1d0273cd4b7ddf348feff5f46b6eb114ea04c9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19536/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19536?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19536   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48714    48714           
=======================================
  Hits        44633    44633           
  Misses       4081     4081
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19536?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19536/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.91% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19536?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/19536?src=pr&el=footer). Last update [bc1d027...d472a46](https://codecov.io/gh/pandas-dev/pandas/pull/19536?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!