Remove rule for conj

[mcabbott]

Special case now handled by DiffRules.

However, to delete this, I think this needs a high lower bound on DiffRules. Or it needs an if to check whether DiffRules has the rule yet.

Closes #531

[codecov-commenter]

Codecov Report

Merging #532 (4c0f4b9) into master (ea822d2) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   84.72%   84.81%   +0.09%     
==========================================
  Files           9        9              
  Lines         825      830       +5     
==========================================
+ Hits          699      704       +5     
  Misses        126      126              

Impacted Files	Coverage Δ
src/dual.jl	72.63% <ø> (+0.48%)	⬆️

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