Mixin cleanup

[SchrodingersGat]

Removes references to the defunct "SettingsContentMixin" plugin class.

This is not used anywhere any more. But, any plugins which call this will need to be adjusted.

[netlify]

✅ Deploy Preview for inventree-web-pui-preview canceled.

Name	Link
🔨 Latest commit	b55a3dd
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/67b966bd632dd70008a54cf6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.07%. Comparing base (44cca7d) to head (b55a3dd).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9142      +/-   ##
==========================================
- Coverage   86.09%   86.07%   -0.02%     
==========================================
  Files        1178     1179       +1     
  Lines       52048    52079      +31     
  Branches     2171     2171              
==========================================
+ Hits        44810    44829      +19     
- Misses       6679     6691      +12     
  Partials      559      559              

Flag	Coverage Δ
backend	88.11% <100.00%> (+<0.01%)	⬆️

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