Update coloring.jl (#215)

Remove a useless call to _sort

Author: amontoison

src/coloring.jl

@@ -365,7 +365,6 @@ function _prevent_cycle!(
     id = find_root!(forest, wx)  # The edge wx belongs to the 2-colored tree T, represented by an edge with an integer ID
     (p, q) = first_visit_to_tree[id]
     if p != v  # T is being visited from vertex v for the first time
-        vw = _sort(v, w)
         first_visit_to_tree[id] = (v, w)
     elseif q != w  # T is connected to vertex v via at least two edges
         forbidden_colors[color[x]] = v