Core: Allow replace/remove of mocked responses

[bblommers]

Fixes #8449

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.63%. Comparing base (4c1321a) to head (514c8e1).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
moto/core/responses_custom_registry.py	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8499      +/-   ##
==========================================
- Coverage   92.63%   92.63%   -0.01%     
==========================================
  Files        1224     1224              
  Lines      105925   105940      +15     
==========================================
+ Hits        98123    98136      +13     
- Misses       7802     7804       +2     

Flag	Coverage Δ
servertests	27.74% <13.33%> (-0.01%)	⬇️
unittests	92.60% <73.33%> (-0.01%)	⬇️

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[ralphie0112358]

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