removes branch from motivation structure

img/motivation-structure.dot

 digraph {
     node [shape = "box", style = "filled, rounded"]
 
-    subgraph clustermaster {
-        label = "master"
+    master4[label = "Dirk (today)\nupdates methods chapter", color = limegreen]
+    master3[label = "Christian (yesterday)\nfixes typos", color = darkorchid1]
+    master2[label = "Dirk (three days ago)\nadds methods chapter", color = limegreen]
+    master1[label = "...", color = grey]
 
-        node [color = limegreen]
-
-        master4[label = "Dirk (today)\nupdates methods chapter", color = limegreen]
-        master3[label = "Christian (yesterday)\nfixes typos", color = darkorchid1]
-        master2[label = "Dirk (three days ago)\nadds methods chapter", color = limegreen]
-        master1[label = "...", color = grey]
-
-        master4 -> master3 -> master2 -> master1
-    }
+    master4 -> master3 -> master2 -> master1
 }