Remove chainmap_impl compat code

[jbrockmendel]

Also avoid having a dependency in compat on core.

[codecov]

Codecov Report

Merging #25840 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            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%)	⬆️
#single	41.83% <100%> (+0.06%)	⬆️

Impacted Files	Coverage Δ
pandas/compat/chainmap.py	66.66% <100%> (+4.76%)	⬆️
pandas/compat/numpy/function.py	87.91% <100%> (ø)	⬆️
pandas/util/testing.py	89.3% <0%> (-0.11%)	⬇️

---

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[codecov]

Codecov Report

Merging #25840 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            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%)	⬆️
#single	41.83% <100%> (+0.06%)	⬆️

Impacted Files	Coverage Δ
pandas/compat/chainmap.py	66.66% <100%> (+4.76%)	⬆️
pandas/compat/numpy/function.py	87.91% <100%> (ø)	⬆️
pandas/util/testing.py	89.3% <0%> (-0.11%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0d3e0e6...7b61159. Read the comment docs.

[jreback]

thanks!