chore: drop 10 years deprecated interface

[nijel]

Proposed changes

I think 10 years of depreciation and getting a warning should be more than enough.

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• Lint and unit tests pass locally with my changes
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• I have added documentation to https://github.com/python-social-auth/social-docs

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.94%. Comparing base (326d4b3) to head (ac24c44).
Report is 56 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1002      +/-   ##
==========================================
+ Coverage   77.88%   77.94%   +0.05%     
==========================================
  Files         347      347              
  Lines       10669    10693      +24     
  Branches      504      455      -49     
==========================================
+ Hits         8310     8335      +25     
+ Misses       2200     2198       -2     
- Partials      159      160       +1     

Flag	Coverage Δ
unittests	77.94% <ø> (+0.05%)	⬆️

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