Improve code coverage

[odow]

x-ref #330 (comment)

These methods are not necessary:

• function_constants is used only when getting the function, where as we get the A matrix directly.

• dot_coefficients is used only when getting the variable dual, which SCS doesn't support: https://github.com/jump-dev/MathOptInterface.jl/blob/ec8434e5a4813c26a05ce6300b2591b36b82b6f1/src/Utilities/results.jl#L413-L422

• set_dot is used only when computing the dual objective value via the fallback, which we compute directly.

Now there's a different argument for keeping these if/when the set gets added to MOI proper (#330 (comment)), but SCS.jl is not the place for them.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.34%. Comparing base (333120f) to head (a242092).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   92.16%   96.34%   +4.17%     
==========================================
  Files          12       12              
  Lines         689      656      -33     
==========================================
- Hits          635      632       -3     
+ Misses         54       24      -30     

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