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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.81%. Comparing base (ea822d2) to head (4c0f4b9).
⚠️ Report is 100 commits behind head on master.

Additional details and impacted files

@@ 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 

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