Revert "[assembler.wrapper] adds syncing in matrix constraints"

This reverts commit 0c71028be4dc41294c8fdf840913a67fa549bd5a.
I was still having issues with this despite the locking.
So I switched back to not assembling constraints in parallel.
[assembler.system] fixes a a name shadowing warning
[elliptic-sipdg-discretization] readd current_grid_width()
- without it, compiler complains about pure virtual function
[elliptic-sipdg-discretization] readd current_grid_width()
- without it, compiler complains about pure virtual function
[test.products_boundaryl2/elliptic] fix unsafe conversion warning
[test.products_boundaryl2/elliptic] fix unsafe conversion warning
[test.elliptic-s(w)ipdg-discretization] drop dune-fem dependency
-instead of using Fem::GridWidth::calcGridWidth do calculation locally
[test.elliptic-s(w)ipdg-discretization] calculate grid_width from one entity only
[test.elliptic-s(w)ipdg-discretization] use entity_diameter to calculate grid width