The `groups_map` array (which indicates the group an element belongs
to) is only intended to reduce loops when creating groups, speeding up
that part of the code.
But on the downside, it requires more memory usage, and adds one more
step to access groups (`groups_offs[groups_map[i]]` instead of just
`groups_offs[i]`).
So removing that array decreases memory usage and speeds up access in
another part of the code.
The profile showed a +6% advantage when the Weld modifier affects large
portions of the mesh.
But it also showed a loss of around -0.6% in performance for cases
where the Weld modifier affects small parts of the mesh.
The biggest benefit is to save memory and speed up some cases.
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