Improve PolynomialSequence.connection_graph()

Simplify the code; also, the previous code has to iterate over variables
of the sequence twice (this is really bad before #35510)

src/sage/rings/polynomial/multi_polynomial_sequence.py

@@ -908,15 +908,10 @@ def connection_graph(self):
             sage: F.connection_graph()
             Graph on 3 vertices
         """
-        V = sorted(self.variables())
         from sage.graphs.graph import Graph
         g = Graph()
-        g.add_vertices(sorted(V))
         for f in self:
-            v = f.variables()
-            a,tail = v[0],v[1:]
-            for b in tail:
-                g.add_edge((a,b))
+            g.add_path(f.variables())
         return g
 
     def connected_components(self):