Remove ConstraintName for VariableIndex constraints

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.62%. Comparing base (0d0f0dd) to head (c512e5b). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #247 +/- ## ========================================== - Coverage 63.77% 63.62% -0.15% ========================================== Files 6 6 Lines 3511 3486 -25 ========================================== - Hits 2239 2218 -21 + Misses 1272 1268 -4 ```

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