[SofaMiscFem] Remove unnecessary copy/pasted code

[alxbilger]

TriangleFEMForceField::addToMatrix was a copy/paste of ForceField::addToMatrix, which is a base class: it is unnecessary.
Note it is used in TriangleFEMForceField::addKToMatrix.

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I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[alxbilger]

[ci-build][with-all-tests]