refactor(topology): dedupe Segment nnodes and exports

Use the generic `AbstractTopology{N}` node count, fix LICENSE URL, and remove
duplicate `Seg*` exports.

- Drop redundant `nnodes`/`export` lines from `segments.jl`.

src/topology/segments.jl

@@ -1,5 +1,5 @@
 # This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 """
     Segment{N} <: AbstractTopology{N}
@@ -15,7 +15,6 @@ struct Segment{N} <: AbstractTopology{N} end
 const Seg2 = Segment{2}
 const Seg3 = Segment{3}
 
-nnodes(::Segment{N}) where {N} = N
 dim(::Segment{N}) where {N} = 1
 
 function reference_coordinates(::Segment{2})
@@ -53,4 +52,3 @@ nvertices(::Segment) = 2
 nedges(::Segment) = 1
 nfaces(::Segment) = 2  # Two endpoints
 
-export Seg2, Seg3