refactor: Remove vendor package dependencies (HeatTransfer, FEMBeam, Mortar*)

Removed all vendor package imports to make JuliaFEM standalone:
- Commented out: HeatTransfer, FEMBeam, MortarContact2D, MortarContact2DAD
- Commented out: AbaqusReader, AsterReader
- These will be consolidated later or remain as vendor archives

Philosophy shift: Focus solely on making JuliaFEM.jl work standalone
- No need for backward compatibility with abandoned vendor packages
- Vendor packages are archaeological artifacts, not dependencies

Result:
- ✅ JuliaFEM still loads successfully
- ✅ Exports reduced to 122 symbols (down from 134)
- Tests status unchanged (still investigating core issues)

Next: Fix actual code issues, not vendor compatibility

src/JuliaFEM.jl

@@ -162,10 +162,11 @@ import Base: getindex, setindex!, convert, length, size, isapprox,
    ==, +, -, *, /, haskey, copy, push!, isempty, empty!,
    append!, read, copy
 
-using AbaqusReader
-using AsterReader
+# TODO: Consolidate these vendor packages later
+# using AbaqusReader
+# using AsterReader
 
-@reexport using HeatTransfer
+# @reexport using HeatTransfer
 include("problems_elasticity.jl")
 export Elasticity
 include("materials_plasticity.jl")
@@ -175,13 +176,14 @@ export Dirichlet
 
 export assemble!, postprocess!
 
+# TODO: Consolidate vendor packages (FEMBeam, Mortar) later
 # Structural elements: beams
-@reexport using FEMBeam
+# @reexport using FEMBeam
 
 ### Mortar methods ###
 
-@reexport using MortarContact2D
-@reexport using MortarContact2DAD
+# @reexport using MortarContact2D
+# @reexport using MortarContact2DAD
 
 include("problems_mortar.jl")
 include("problems_mortar_3d.jl")