Revert "Solve issue when inserting a point along a non-manifold edge with 2 singular extremities"

[corentin-prigent]

Reverts MmgTools/mmg#287, as it creates unwanted configurations in mmg3d.

[mpotse]

Thanks! I patched it and now I can refine my meshes again. The refusal to split an edge between two Corners led to very bad results.