Changes

[Constraint.Lagrangian.Solver] Fix assert in GenericConstraintSolver (#4389)

* [Constraint.Lagrangian.Solver] Fix assert in GenericConstraintSolver

The construction of xId caused an assert when `cParams->constOrder() == sofa::core::ConstraintOrder::VEL`. It happens only in debug, and it had no consequence because it was not used.

* Simpler solution

* Remove unnecessary include

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Co-authored-by: Paul Baksic <30337881+bakpaul@users.noreply.github.com>