chore(io): remove unused JuliaFEM.IO submodule stub

Delete the nested `IO` module that only re-exported the retired Abaqus glue;
the package entry point now includes `gmsh_reader.jl` directly.

- Remove `src/io/io.jl` (SparseArrays/Logging imports and `abaqus_reader`
  forwarding).

src/io/io.jl

@@ -1,42 +0,0 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
-
-"""
-    JuliaFEM.IO
-
-Input/Output submodule for reading mesh files and writing results.
-
-# Available (zero external dependencies)
-- `abaqus_read_mesh(filename)`: Read ABAQUS .inp files
-- `create_surface_elements(mesh, name)`: Create surface elements
-- `create_nodal_elements(mesh, name)`: Create nodal elements
-
-# Future (requires optional dependencies)
-- Code Aster .med files: `aster_read_mesh` (requires HDF5.jl)
-- Xdmf output format: `Xdmf` writer (requires HDF5.jl + LightXML.jl)
-- VTK output: Native Julia implementation (no dependencies)
-
-# Usage
-```julia
-using JuliaFEM
-mesh = abaqus_read_mesh("model.inp")
-body = create_elements(mesh, "BODY")
-```
-"""
-module IO
-
-using ..JuliaFEM: Element, Mesh
-
-# Re-export for convenience
-import ..JuliaFEM.Preprocess
-using SparseArrays, LinearAlgebra
-using Logging
-
-include("abaqus_reader.jl")
-export abaqus_read_mesh, create_surface_elements, create_nodal_elements
-
-# AsterReader would go here when HDF5 is added as optional dependency
-# include("aster_reader.jl")
-# export aster_read_mesh
-
-end # module IO