chore(test): delete retired DOF system suite

Remove tests targeting the removed `DOFManager`-style API.

- Drop `test/dofs/test_dof_system.jl`.

test/dofs/test_dof_system.jl

@@ -1,66 +0,0 @@
-using JuliaFEM
-using Tensors
-
-# Create simple 2-element mesh (2 hexes sharing a face)
-# Hex 1: nodes 1-8
-# Hex 2: nodes 5-8, 9-12 (shares face 5-6-7-8 with Hex 1)
-
-nodes = [
-    Vec{3}((0.0, 0.0, 0.0)),  # 1
-    Vec{3}((1.0, 0.0, 0.0)),  # 2
-    Vec{3}((1.0, 1.0, 0.0)),  # 3
-    Vec{3}((0.0, 1.0, 0.0)),  # 4
-    Vec{3}((0.0, 0.0, 1.0)),  # 5
-    Vec{3}((1.0, 0.0, 1.0)),  # 6
-    Vec{3}((1.0, 1.0, 1.0)),  # 7
-    Vec{3}((0.0, 1.0, 1.0)),  # 8
-    Vec{3}((0.0, 0.0, 2.0)),  # 9
-    Vec{3}((1.0, 0.0, 2.0)),  # 10
-    Vec{3}((1.0, 1.0, 2.0)),  # 11
-    Vec{3}((0.0, 1.0, 2.0)),  # 12
-]
-
-connectivity = [
-    NTuple{8,UInt32}((1, 2, 3, 4, 5, 6, 7, 8)),
-    NTuple{8,UInt32}((5, 6, 7, 8, 9, 10, 11, 12)),
-]
-
-element_sets = Dict{Symbol,Set{UInt32}}(:all => Set(UInt32[1, 2]))
-
-mesh = Mesh{8,Hexahedron{8}}(nodes, connectivity, element_sets)
-
-println("Mesh created:")
-println("  Nodes: ", length(nodes))
-println("  Elements: ", length(connectivity))
-
-# Create elements with 3D displacement DOFs (Vec{3} at vertices)
-println("\nCreating elements with DOF{Vec{3}, Vertex}...")
-
-ElemType = Element{Hexahedron{8}, Lagrange{1}, DOF{Vec{3}, Vertex}}
-elements, mgr = create_elements!(mesh, ElemType)
-
-println("Elements created:")
-println("  Element count: ", length(elements))
-println("  Total DOFs: ", mgr.total_dofs)
-println("  Expected DOFs: ", 3 * length(nodes), " (3 per node)")
-
-# Check DOF indices for first element
-println("\nElement 1 DOF indices:")
-println("  ", elements[1].dof_indices)
-println("  Length: ", length(elements[1].dof_indices))
-println("  Expected: 24 (8 nodes × 3 DOFs)")
-
-# Check DOF indices for second element (should reuse nodes 5-8)
-println("\nElement 2 DOF indices:")
-println("  ", elements[2].dof_indices)
-println("  Length: ", length(elements[2].dof_indices))
-println("  Expected: 24 (8 nodes × 3 DOFs)")
-
-# Verify node-to-DOF mapping
-println("\nNode-to-DOF mapping:")
-for node_id in [1, 5, 9]
-    dofs = get_node_dofs(mgr, node_id)
-    println("  Node $node_id: DOFs $dofs")
-end
-
-println("\n✓ DOF system test complete!")