MAINT: Remove assert_equal from testing.py
Title is self-explanatory.

Partially addresses #15990.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=h1) Report
> Merging [#16017](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8daf9a7e45344dc5a247410e037dccb41b97a3db?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/16017/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16017      +/-   ##
==========================================
+ Coverage   90.99%      91%   +<.01%     
==========================================
  Files         154      154              
  Lines       50484    50482       -2     
==========================================
- Hits        45940    45939       -1     
+ Misses       4544     4543       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <ø> (ø)` | :arrow_up: |
| #single | `40.44% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.97% <ø> (-0.04%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16017?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/16017?src=pr&el=footer). Last update [8daf9a7...e808a91](https://codecov.io/gh/pandas-dev/pandas/pull/16017?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung can you rebase, I had merged the ``.formats`` move @jreback , @jorisvandenbossche : Rebased, all green, and ready to merge. thanks!