Remove chainmap_impl compat code
Also avoid having a dependency in `compat` on `core`.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=h1) Report
> Merging [#25840](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0d3e0e68cc4dbe0620008d98274d0004dbf5a734?src=pr&el=desc) will **increase** coverage by `0.13%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25840/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25840      +/-   ##
==========================================
+ Coverage    91.3%   91.44%   +0.13%     
==========================================
  Files         173      172       -1     
  Lines       53004    52922      -82     
==========================================
- Hits        48397    48395       -2     
+ Misses       4607     4527      -80
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.01% <100%> (+0.13%)` | :arrow_up: |
| #single | `41.83% <100%> (+0.06%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/chainmap.py](https://codecov.io/gh/pandas-dev/pandas/pull/25840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9jaGFpbm1hcC5weQ==) | `66.66% <100%> (+4.76%)` | :arrow_up: |
| [pandas/compat/numpy/function.py](https://codecov.io/gh/pandas-dev/pandas/pull/25840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9mdW5jdGlvbi5weQ==) | `87.91% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25840/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25840?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/25840?src=pr&el=footer). Last update [0d3e0e6...7b61159](https://codecov.io/gh/pandas-dev/pandas/pull/25840?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/25840?src=pr&el=h1) Report
> Merging [#25840](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0d3e0e68cc4dbe0620008d98274d0004dbf5a734?src=pr&el=desc) will **increase** coverage by `0.13%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25840/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25840      +/-   ##
==========================================
+ Coverage    91.3%   91.44%   +0.13%     
==========================================
  Files         173      172       -1     
  Lines       53004    52922      -82     
==========================================
- Hits        48397    48395       -2     
+ Misses       4607     4527      -80
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.01% <100%> (+0.13%)` | :arrow_up: |
| #single | `41.83% <100%> (+0.06%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/chainmap.py](https://codecov.io/gh/pandas-dev/pandas/pull/25840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9jaGFpbm1hcC5weQ==) | `66.66% <100%> (+4.76%)` | :arrow_up: |
| [pandas/compat/numpy/function.py](https://codecov.io/gh/pandas-dev/pandas/pull/25840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9mdW5jdGlvbi5weQ==) | `87.91% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25840/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25840?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/25840?src=pr&el=footer). Last update [0d3e0e6...7b61159](https://codecov.io/gh/pandas-dev/pandas/pull/25840?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!