[FEM.Elastic] Fix small deformation in TriangleFEMForceField

[alxbilger]

_rotatedInitialElements is not used in the small method. It is not resized (size = 0), therefore it leads to a crash. This instruction has been introduced in https://github.com/sofa-framework/sofa/pull/2287. I reverted this line only by the original code.

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[alxbilger]

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