[FEM.Elastic] Remove complex unproductive stiffness matrix data structure

[alxbilger]

Since #6069, it is suspected that the ElementStiffnessMatrix data structure did not achieve its performance objectives. It was suggested to replace it by a simple matrix, and contain them in a vector.

This new PR applies the suggestion, and removes totally the complex data structure, that is no longer used.

[with-all-tests]

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[fredroy]

@alxbilger did you run benchmarks on this branch vs master ?
I can do it on macOS and linux if you want. (to compare with #6069)

[fredroy]

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

[alxbilger]

@fredroy For now, I tested with https://github.com/alxbilger/Elasticity/blob/master/benchmark/python/run_tetra.py and it speed it up