ref: remove OptionMixin

[asottile-sentry]

this fixes types for the various models involved

mixins aren't sound in typing unless they have an associated protocol -- and even then there aren't dependent types so this mixin couldn't possibly be typed properly. it also wasn't being used at runtime either so it was easier to just write the methods directly in the two subclasses

[JoshFerge]

👏

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.04%. Comparing base (eca152a) to head (c7c6374).
Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #72797      +/-   ##
==========================================
- Coverage   78.05%   78.04%   -0.01%     
==========================================
  Files        6601     6602       +1     
  Lines      294329   294356      +27     
  Branches    50751    50746       -5     
==========================================
+ Hits       229729   229741      +12     
- Misses      58361    58376      +15     
  Partials     6239     6239              

Files	Coverage Δ
src/sentry/models/options/option.py	100.00% <100.00%> (ø)
src/sentry/models/project.py	94.75% <100.00%> (+0.03%)	⬆️
...sentry/services/hybrid_cloud/organization/model.py	90.47% <100.00%> (-0.05%)	⬇️
src/sentry/services/hybrid_cloud/project/model.py	63.63% <100.00%> (-1.07%)	⬇️
src/sentry/models/organization.py	87.60% <87.50%> (-0.11%)	⬇️

... and 20 files with indirect coverage changes

[markstory]

Looks good to me.